Jointly funded by the U.S. Geological Survey and U.S. Bureau of Land Management 

Mercury Release from the Rathburn Mine, 

Petray Mine, and Bear Valley Saline Springs, 

Colusa County, California 2004–2006 

By Aaron J. Slowey and James J. Rytuba 

Open-File Report 2008-1179 

U.S. Department of the Interior 

U.S. Geological Survey 
U.S. Department of the Interior 

DIRK KEMPTHORNE, Secretary 

U.S. Geological Survey 

Mark D. Myers, Director 

U.S. Geological Survey, Reston, Virginia 2008 

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Slowey, A.J., Rytuba, J. J., 2008, Mercury release from the Rathburn Mine, Petray Mine, and 

Bear Valley saline springs, Colusa County, California 2004-2006: U.S. Geological Survey, OpenFile Report 2008-1179, 49 p. [http://pubs.usgs.gov/of/2008/1179/]. 

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ii 
Contents 

Figures............................................................................................................................................iii 

Tables............................................................................................................................................. iv 

Abbreviations, definitions, and datum used.................................................................................... v 

1. Introduction................................................................................................................................. 1 

1.1 Background and objectives................................................................................................... 1 

1.2 Geology................................................................................................................................. 3 

1.3 Hydrology ............................................................................................................................. 3 

1.4 Physicochemical and biogeochemical considerations for mercury in Bear Valley.............. 8 

2. Sample locations and methods.................................................................................................. 11 

2.1 Sample locations and conditions......................................................................................... 11 

2.2 Sample handling.................................................................................................................. 11 

2.2.1 Rock, tailings, and stream sediment ............................................................................ 11 

2.2.2 Water............................................................................................................................ 14 

2.3 Analytical methods ............................................................................................................. 14 

2.3.1 Sediment and tailings................................................................................................... 14 

2.3.2 Water............................................................................................................................ 16 

3. Results and discussion .............................................................................................................. 17 

3.1 Tailings and dry sediment at the Rathburn and Petray Mines ............................................ 20 

3.2 Surface and saline ground water seepage ........................................................................... 21 

4. Conclusions............................................................................................................................... 36 

4.1 General conclusions............................................................................................................ 36 

4.2 Predicting the efficacy of a removal of Rathburn and Petray Mine waste ......................... 38 

6. References................................................................................................................................. 38 

Appendix 1. Photographs.............................................................................................................. 42 

Figures 

Figure 1. Location of Bear Valley, Colusa County, California. ..................................................... 2 

Figure 2. Petray and Rathburn Mine and Bear Valley sample locations..................................... 4-6 

Figure 3. Physicochemical model of mercury transport from Rathburn and Petray Mines and 

within Bear Valley. ................................................................................................................. 9 

Figure 4. Biogeochemical model for processes putatively affecting the transport of mercury in 

the Bear Creek basin. Sources of bicarbonate include dissolution of CO2 gas evolved from 

connate ground water seeps and travertine dissolution during acidic rainfall...................... 10 

Figure 5. Acid titration curves for filtered (<0.45 μm) Rathburn and Petray Mine drainage 

sampled in May 2005 (a) and Bear Valley saline spring-influenced surface water sampled in 

December 2006 (b). .............................................................................................................. 25 

Figure 6. Correlation between log10 transforms of Na and Cl in filtered (<0.45 μm) spring, pore, 

and surface water in streams draining the Petray Mine and Bear Valley. ............................ 26 

Figure 7. (a) Excitation-emission matrix for Bear Valley ground water seepage location 06RP1. 

(b) Fluorescence emission spectra at 370 nm excitation wavelength, as indicated by the 

dashed line in (a)................................................................................................................... 33 

Figure 8. Apparent correlation between DOC the fluorescence index and SUVA254. ................. 34 

iii 
Figure 9. Correlation between log10 transforms of filtered (<0.45 μm) and total Hg in surface 

water in streams draining the Petray Mine and Bear Valley. ............................................... 34 

Figure 10. Petray Mine, May 2005 (denoted as photo 10 in fig. 2).............................................. 42 

Figure 11. Near location M, looking eastward toward Bear Valley, May 2005 (denoted as photo 

11 in fig. 2)............................................................................................................................ 42 

Figure 12. Location M, May 2005 (denoted as photo 12 in fig. 2)............................................... 43 

Figure 13. South fork of tributary 1, May 2005 (denoted as photo 13 in fig. 2). ......................... 43 

Figure 14. Algal biomass in Dead Shot Creek, May 2005 (denoted as photo 14 in fig. 2).......... 44 

Figure 15. Cinnabar ore in alterred serpentine at the Petray Mine, May 2005............................. 44 

Figure 16. Alterred serpentine at the Rathburn Mine (a), ppen pit at the Petray Mine (b), Petray 

fault zone (c), north Pit at the Petray Mine (d), waste rock at the Rathburn Mine (e), alterred 

serpentine at the Petay Mine (f), silica-carbonate veins at the Petray Mine (g), and terraced 

topography at the Petray Mine (h). (taken May 2005).......................................................... 45 

Figure 17. Travertine along tributary 1, December 2004. ............................................................ 46 

Figure 18. Travertine “falls” along tributary 1, December 2004.................................................. 46 

Figure 19. Ground-level view of vegetated land along fault zone depicted in figure 2, December 

2006....................................................................................................................................... 47 

Figure 20. Large seep of connate ground water, including evolving gas (presumably CO2, based 

on its lack of any detectable odor), December 2006............................................................. 47 

Figure 21. Unsaturated carbonate-rich mud and grass coated with efflorescent salt, December 

2006....................................................................................................................................... 48 

Figure 22. Carbonate and iron-rich ground water seepage. Note also faint regions of green 

material and the nonuniformity of iron (orange) and green coloration, December 2006. .... 48 

Figure 23. Tributary 1 on December 13, 2006 at dusk (photo levels enhanced). Note white 

mineral coating along stream channel................................................................................... 49 

Figure 24. Ground water seepage and resulting drainage channel. Shiny object is an aluminum 

foil-covered pore water sampling device, December 2006. ................................................. 49 

Tables 

Table 1. Altered serpentinite, calcine and sediment sample locations at open pits at the Rathburn 

and Petray Mines................................................................................................................... 12 

Table 2. Stream water and sediment sample locations, quantities, and dates............................... 13 

Table 3. Elemental composition of tailings, waste rock, and sediment at the Rathburn and Petray 

Mine sites.............................................................................................................................. 18 

Table 4. Hg and methyl Hg in surface water, sediment, and porewater and sulfide in pore water.

............................................................................................................................................... 23 

Table 5. Predicted proportions of saline ground water in water samples based on Cl 

concentration in spring, pore, and surface waters................................................................. 28 

Table 6. Temperature, pH, alkalinity, and concentrations of selected anions. ............................. 31 

Table 7. Concentrations and properties of dissolved organic matter in saline ground water 

seepage sampled in Bear Valley, December 2006................................................................ 32 

iv 
Abbreviations, definitions, and datum used 

Vertical and horizontal coordinate information is referenced to the North American Datum of 

1927 (NAD 27 CONUS). 

aq, dissolved aqueous species (operationally defined as the filtered water fraction). 

DOC, dissolved organic carbon 

DOM, dissolved organic matter 

EE/CA, Engineering evaluation/cost analysis for “non-time-critical removal actions,” as defined 

by the U.S. Environmental Protection Agency’s Comprehensive Environmental Response, 

Compensation, and Liability Act (CERCLA). 

EEM, Excitation-emission matrix of ultraviolet fluorescence of dissolved organic carbon. 

FI, fluorescence index of dissolved organic carbon, computed by dividing the emission intensity 

at 470 nm (I470) by that at 520 nm (I520) at an excitation wavelength of 370 nm. 

Hg, generic shorthand for mercury; does not denote speciation. 

HgT, total mercury (inorganic plus organic). 

HgF, total mercury (inorganic plus organic) in a filtered sample (either 0.1 μm or 0.45 μm, 

as specified in the text). 

Hg(II), compounds containing divalent inorganic mercury (Hg2+) and anionic ligands (for 

example, Cl-, S2-). 

meq/L, milliequivalents (10-3 charge equivalents) per liter. 

MMHg, monomethyl mercury, methylmercury, and monomethylmercury (CH3Hg+

) 

ng/g, nanogram per gram, equivalent to one (1) part per trillion. 

ng/L, nanogram per liter, approximately equivalent to one (1) part per trillion. 

ppm, parts per million, equivalent to mg/kg or μg/g. 

s, sample standard deviation 

SC, specific (electrical) conductivity, reported in units of millisiemens per centimeter (mS/cm). 

SUVA254, Specific ultraviolet absorbance, calculated by dividing the UV absorbance at a 

wavelength of 254 nm by the DOC concentration and reported in the units of L·mg C-1 m

-1. 

v/v, volume per volume 

v 
1 

 

Mercury Release from the Rathburn Mine, Petray Mine, and Bear 

Valley Saline Springs, Colusa County, California 2004-2006 

Aaron J. Slowey* and James J. Rytuba† 

1. Introduction 

1.1 Background and objectives 

The Rathburn and Petray mercury (Hg) deposits are the northernmost and youngest Hg deposits 

in the Coast Range Hg mineral belt and are located in the Bear Creek portion of the Cache Creek 

watershed, Colusa County (fig. 1). The Rathburn Hg mine is relatively small, having produced 

100 flasks of Hg, primarily in the period from 1892-1893. More recent mining in the late 1960s 

and early 1970s recovered about 400 flasks of Hg from the Petray open pit mines located 

immediately to the north of the Rathburn Mine. The Petray open pits caused considerable surface 

disturbance and contributed mine waste material to the headwaters of several small tributaries to 

Bear Creek located to the east. The mines are located on federal land managed by the U.S. 

Bureau of Land Management (BLM). The BLM requested that the U.S. Geological Survey 

(USGS) measure and characterize Hg and geochemical constituents in tailings, sediment, and 

water at the Rathburn and Petray Mines, tributaries draining the mine area, and within Bear 

Valley west of Bear Creek (fig. 2). This request was made in response to a California State 

Water Board mandate that the BLM conduct an Engineering Evaluation/Cost Analysis (EE/CA)

of “non-time-critical removal actions.” The EE/CA applies to removal of Hg-contaminated mine 

waste from the Rathburn and Petray Mines as a means of reducing Hg transport to Bear Creek. 

The mandate presupposes that the Rathburn and/or Petray Mines are the primary sources of Hg 

to Bear Creek, although we consider this assumption to be a question. We also consider the 

question of how to directly evaluate the efficacy of eliminating the mines as Hg sources, should 

they be remediated. 

Accordingly, the following are the objectives of this study: 

i. Determine all potential natural and anthropogenic sources of Hg to Bear Creek. 

ii. Evaluate the extent to which Hg is transported to Bear Creek from all sources. 

iii. Identify physicochemical and biogeochemical processes affecting the transport of Hg based 

on existing (preremediation) field conditions. 

* U.S. Geological Survey, 345 Middlefield Rd, MS 901, Menlo Park, California 94025; aslowey@usgs.gov; 

650 329-5474. 

†

jrytuba@usgs.gov; 650 329-5418. 
Figure 1. Location of Bear Valley, Colusa County, California. 

This report summarizes data obtained from field sampling of mine tailings and waste rock at the 

Rathburn and Petray Mines that was initiated in July 17, 2001 and water and sediment in 

regional springs and tributaries that drain from the mine area into Bear Creek on December 14, 

2004 and February 16 and May 27, 2005. Although it was initially assumed that the mines were 

the cause of elevated levels of monomethyl Hg measured by the Central Regional Water Quality 

Control Board in tributaries near their confluence with Bear Creek (Foe and others, unpublished 

results), it became apparent during this study that ground water springs were also potential 

sources of Hg. In addition to sampling of springs in May 2005, saline ground water seepage 

along an unnamed fault on the west side of Bear Valley was sampled on December 13-14, 2006. 

We did not sample water or sediment in Bear Creek itself during this study. Our results permit a 

preliminary assessment of mining and natural sources of Hg and associated chemical constituents 

that could elevate levels of monomethyl Hg in Bear Creek. 

The Bear Creek drainage basin comprises the northeastern part of the much larger Cache Creek 

basin, which has been the subject of several studies of mercury transport to the San Francisco 

Bay Delta (Bloom, 2001; Domagalski and others, 2004a; Domagalski and others, 2004b; Slotton 

and others, 2004; Cooke and Morris, 2005). We compare our results to some of the findings 

recapitulated by Domagalski and others (2004b) but do not attempt to reconcile our findings with 

these previous studies. Here, we attempt to develop a basis for incorporating biogeochemical and 

hydrological processes into a future monitoring program. A more fully developed conceptual 

model is the objective of a new California Bay Delta Authority-funded project that intends to 

ensure that future monitoring improves our understanding of the ecology of Hg contamination in 

the Cache Creek basin, on the basis of the evaluation of past work. 

2 
1.2 Geology 

Mercury mines in the Bear Creek watershed are located near the major tectonic boundary that 

separates the sedimentary rocks of the forearc Great Valley Sequence from varied lithologies of 

the Franciscan accreted terrain. The Rathburn and Petray Hg deposits are localized along fault 

zones in serpentinite that has been hydrothermally altered and cut by numerous quartz and 

chalcedony veins. Cinnabar occurs as fine-grained masses on fractures and disseminated in lowangle fault and shear zones. The Hg ores formed in a hot spring system in the steam-heated 

environment present above the boiling paleo ground water table. 

Cold saline springs and travertine terraces (sample locations M and MB, fig. 2) and drainages 

with sediment cemented by carbonate minerals (sample locations C, K, and L; see fig. 2 and figs. 

11, 17, and 18 in the appendix) are present to the east of the Petray Mine and represent the 

waning stage of the hydrothermal system that formed the Hg deposits. Numerous saline springs, 

such as Cain springs (sample location CS, fig. 2) are present in the Bear Creek watershed. A 

previously unrecognized north-northwest-trending fault in Bear Valley, here termed the Bear 

Fault, is located 2 kilometers east of the Petray Mine and about 1 kilometer west of Bear Creek 

(fig. 2). Saline springs are localized along the length of the fault zone, exposed by areas of 

vegetation and whitish-gray carbonate-rich material (figs. 19-21). The largest volume of spring 

water is released from the central part of the fault and flows into tributaries 1 and 2 (fig. 2). 

These spring waters are derived from connate fluids within the Great Valley sedimentary rocks 

and consist of evolved Cretaceous seawater having high concentrations of CO2, methane, and asyet unknown hydrocarbons (referred to later as organic carbon). The fluids reach the surface 

along fault zones located within the Bear Creek watershed and account for the unusually high 

chloride, carbonate and sulfate concentrations of Bear Creek and several of its tributaries. 

1.3 Hydrology 

It is hypothesized that there are two sources of Hg to Bear Creek: surface water drainage from 

the Petray and Rathburn Mines and saline ground water from Bear Valley. Each source may 

differ in the speciation of mercury at the point of release. The aqueous geochemistry of each 

source also differs. However, at Bear Creek, aqueous geochemistry will reflect mixing of 

meteoric water and saline ground water to varying degrees, depending on rainfall intensity 

(assuming saline springs dominate ground water flow). It is expected that the aqueous 

geochemistry in Bear Creek will be dominated by that of saline ground water during the dry 

season and by surface runoff during the wet season but transiently and to a limited extent. As a 

result, it is expected that assessing the hydrological (and therefore geochemical) importance of 

surface water drainage from the mined areas relative to saline ground water to the aqueous 

geochemistry of Bear Creek and downstream water bodies (for example, Cache Creek) will be 

difficult without extensive geochemical, geophysical, and hydrological investigation over a 

multi-year period that captures appreciable hydrological variability (see also section 4). 

3 
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Figure 2a 

This figure is optimized for printing on a single sheet of 11” x 17” paper. 

Figure 2. Petray and Rathburn Mine and Bear Valley sample locations 

4 
Placeholder page for 

Figure 2b 

This figure is optimized for printing on a single sheet of 11” x 17” paper. 

5 
Placeholder page for 

Figure 2c 

This figure is optimized for printing on a single sheet of 11” x 17” paper. 

6 
Water inputs to Bear Creek consist of a combination of ground and surface water. Surface water 

at the Petray Mine location is believed to arise from meteoric precipitation and is, therefore, 

ephemeral. Surface water from the north pit of the Petray Mine drains primarily through one 

incised channel, referred to here as the north fork of tributary 1 and shown in figure 2. On the 

basis of topography, surface flow from the middle pit of the Petray Mine flows eastward to the 

south fork of tributary 3 and northeastward into the south fork of tributary 1. A saline spring east 

of the north pit actively releases carbonate-rich water, forming a carbonate terrace (travertine), 

depositing carbonate minerals, and cementing the sediment throughout the downstream channel 

of the south fork of tributary 1. It is unclear where surface drainage from the Rathburn Mine 

location goes, but it may join drainage from the south pit of the Petray Mine and/or drain to 

tributary 2 (fig. 2). Another drainage channel called Dead Shot Creek originates north of the 

Petray Mine. As can be viewed in current aerial photographs of the area, Dead Shot Creek has 

flowed parallel to tributary 1 before merging with it just upstream of the Bear Creek confluence. 

In 2005, however, we observed Dead Shot Creek to merge with tributary 1 much further 

upstream, just downstream of the confluence of the north and south forks of tributary 1. Recent 

debris flow deposits have caused the active channel of Dead Shot Creek to move southward on 

the alluvial fan such that it no longer is accurately represented on the topographic map. 

As stated above, some surface flow in Bear Valley is due to upward percolation of ground water 

originating from connate fluids, some of which are heated and thus geothermal, primarily in the 

Sulphur Creek tributary to Bear Creek. Several 1- to 2-m depressions at the surface of Bear 

Valley were observed in December 2006. Some of these depressions were dry or moist from 

rainfall, but nearly all had narrow and shallow channel depressions leading towards Bear Creek 

and indicative of past discharge (fig. 24). When meteoric input is minimal, surface water is likely 

dominated by seepage of cold saline connate water. 

Seepage of saline ground water appears to change in its spatial occurrence and extent over time. 

Ground water was clearly observable in shallow pits dug for sampling location BF1. In several 

pits dug in the vicinity of location BF2, a hard layer was encountered approximately one foot 

below ground surface. This layer is presumed to be a travertine deposit formed from past spring 

flow. Evidence of past ground water seepage was also observed north of Bear Creek Ranch Road 

in December 2006, but soil in the vicinity of location BF3 (fig. 2) was not saturated at the time. 

Vegetation varies appreciably, presumably depending on the stability of soil as affected by 

ground water inundation. One location (BF4) supported growth of tall plants, most had short 

grasses, and a few lacked vegetation. Several surface depressions were noted along the Bear 

Fault, most of which were not actively seeping water. 

In addition to surface water and runoff of seepage, it is hypothesized that Bear Creek receives a 

substantial input of ground water via direct subsurface transport. Several brief investigations in 

late-2004 (Aug. 5, Sept. 22, and Oct. 26th) by Foe and others revealed that the specific electrical 

conductivity (SC) of Bear Creek water increased by about 10% (roughly 800 to 900 μS/cm) 

downstream of the Bear Valley segment that includes drainage from the Petray and Rathburn 

Mines (Foe, pers. comm.; unpublished data). 

7 
These hydrological features influence the transport and biogeochemical transformations of Hg in 

Bear Valley primarily through the physicochemistry1 of carbonate-rich materials. For example, 

carbonate minerals could be cementing mineral particles to which Hg is likely sorbed. 

Dissolution of carbonate minerals would release particles. In addition, carbonate and other 

constituents of saline ground water, such as dissolved organic carbon (DOC), likely influence 

both geochemical and biological processes that affect the reactivity of inorganic Hg in the 

vicinity of Bear Creek, thus affecting its potential to be methylated and taken up by resident 

biota (Slotton and others, 2004). 

1.4 Physicochemical and biogeochemical2

 considerations for mercury in Bear 

Valley 

Both the drainages from the Petray Mine area and Bear Valley tributaries (1 through 5, fig. 2) to 

Bear Creek are supplied with carbonate from saline springs located throughout the drainage 

basin. Sulfate is also present in both mine drainage and ground water. In addition, ground water 

contains high levels (up to 20 mg/L, as discussed later) of dissolved organic carbon. These 

constituents can strongly influence the environmental geochemistry of Hg (discussed in section 

3.2). Figures 3 and 4 summarize physicochemical and biogeochemical processes that could affect 

the transport and reactivity of mercury in the Bear Valley/Bear Creek system. The following 

paragraphs briefly explain these processes. 

Precipitation of carbonate minerals will cement sediment (including mine waste), such as in the 

south fork of tributary 1, and therefore should retard physical transport of Hg from the Rathburn 

and Petray Mines. However, periods of intense rainfall may sufficiently lower the pH of surface 

water in Bear Creek tributaries to partially dissolve carbonate cements, and dissolve and release 

Hg-bearing particles (fig. 3). As the wet season ends and meteoric input wanes, water seeping 

from the western hills may mix with upwardly flowing saline ground water and provide a base 

flow through the Bear Creek tributaries. This temporary hydrological state may reduce the 

physical and chemical transport of Hg. However, lower flow and warmer temperatures likely 

enhance a number of biologically mediated processes that affect the speciation of Hg and 

formation of monomethyl Hg. Such processes will be related to geochemical conditions 

dominated by carbonate, sulfate, dissolved organic carbon (DOC), and, on the basis of visual 

observations (figs. 12 and 22), iron. 

Figure 4 summarizes a number of biogeochemical processes that may influence the speciation 

and transport of mercury in Bear Valley. Photosynthetic organisms such as algae consume 

carbonate, which can change pH: the change is dependent on the assimilation of other ions 

(Stumm and Morgan, 1996). For example, ammonia (NH4

+

) assimilation as part of 

photosynthesis generates acid, which could sufficiently lower the pH enough to dissolve 

carbonate minerals. Dissolution of carbonate minerals may release Hg-bearing particles. 

Ammonia is highly enriched in active mercury geothermal systems such as at the Sulphur Bank 

mercury deposit where elevated concentrations of NH4

+

 are present in springs (King and Rytuba, 

pers. communication). Sources of ammonia to the Bear Creek watershed are likely saline ground 

water and urine excreted by cattle grazing in the area. The latter source implicates land-use as an 

 

1 In this report, physicochemistry refers to coupled chemical and physical processes (for example, precipitation, 

dissolution, and colloid transport). 

2

 In this report, the term biogeochemical refers to processes mediated by microorganisms as well as abiotic 

geochemical constituents. 

8 
influence on the speciation and mobility of Hg in Bear Valley. The relatively high concentrations 

of nitrate, up to 90 ppm in the most saline spring waters along the Bear Fault, are likely a result 

of the oxidation of NH4

+

 in ground water when it mixes with more-oxygenated meteoric water. 

Figure 3. Physicochemical model of mercury transport from Rathburn and Petray Mines and within Bear 

Valley. 

Sulfur cycling is important to Hg speciation and transport due to the ubiquity of sulfate-reducing 

microbial activity in subsurface and near-surface environments (Gibson, 1990; Rabus and others, 

2006) and the strong chemical affinity between reduced sulfur species and Hg(II) (Hepler and 

Olofsson, 1975; Paquette and Helz, 1997; Jay and others, 2000; Brandon and others, 2001). As 

shown in figure 4, reduction of sulfate to form bisulfide is only one of several potentially 

important reactions. In particular, oxidation of sulfide [S(-II)] by microorganisms such as 

Beggiatoa and Thioploca spp. (Teske and Nelson, 2006) and abiotic oxidation of S(-II) by Fe(III) 

and Mn(IV) (hydr)oxides to elemental sulfur [S(0)] (Thamdrup and others, 1994; Poulton and 

others, 2004) introduces reaction mechanisms that can release mercury from species such as 

cinnabar (α-HgS(s)) or authigenic metacinnabar (β-HgS(s)) (Slowey and Brown Jr., 2007). The 

roles of organic carbon—an abundant constituent in saline ground water within Bear Valley—

may include dissolution of α− and β−HgS(s) (Ravichandran and others, 1998; Waples and others, 

2005), which may be photochemically influenced. Organic carbon can also limit the nucleation 

and crystal growth of β−HgS(s) in the presence of sulfide (Ravichandran and others, 1999).

The mining sources of mercury are small in spatial extent compared to the transport pathways in 

Bear Valley. As a result, Hg from the mines is expected to be widely dispersed. Saline ground 

water may also contribute Hg to Bear Creek via seepage/surface runoff and subsurface flow. 

9 
Understanding the net transport of Hg from the mines to Bear Creek is addressed to a limited 

extent by this brief field investigation. To understand whether there is a net flux of Hg to Bear 

Creek, seasonal, multi-year monitoring of Hg in water and sediment in the Bear Creek tributaries 

and in Bear Creek downstream of the confluences of these tributaries is required. These 

considerations for Hg transport to Bear Creek are developed in light of the results of this study 

and are recapitulated in section 4. 

Subsurface flow, seepage, and runoff of ground water throughout Bear Valley are expected to 

strongly influence the aqueous chemistry of Bear Creek. Since Bear Creek is a small part of the 

Cache Creek watershed, the spatial extent to which ground water influences the geochemistry of 

downstream waterways is unclear. However, the larger Cache Creek basin includes several other 

sources of saline ground water that, to our knowledge, have yet to be characterized or 

enumerated. 

Figure 4. Biogeochemical model for processes putatively affecting the transport of mercury in the Bear 

Creek basin. Sources of bicarbonate include dissolution of CO2 gas evolved from connate ground water 

seeps and travertine dissolution during acidic rainfall. 

10 
2. Sample locations and methods 

2.1 Sample locations and conditions 

Samples were collected to assess the concentration of Hg and biogeochemically relevant 

constituents in rocks within the open pits at the mine (altered serpentinite) and in tailings in 

waste rock piles at the Rathburn and Petray mines (figs. 10, 15, 16). Water, soil, and sediment 

were sampled along Dead Shot Creek (fig. 13), stream channels (referred to as tributaries 1 

through 5) originating near the Petray open-pit mine and coursing through Bear Valley toward 

Bear Creek (fig. 23), and along an inferred fault zone in Bear Valley marked by numerous 

saline springs. Tables 1 and 2 detail the various types of samples collected, locations, and 

collection dates during the Winter 2004, February 2005, May 2005, and December 2006 periods. 

Field locations are shown on figure 2. During the December 2004 period, stream flow was low 

with no precipitation; in February 2005, sampling occurred during a storm that resulted in higher 

stream flow; the weather was stable and no precipitation occurred in May 2005; and while 

rainfall did occur over days prior to the December 2006 sampling period, precipitation while 

sampling was negligible and surface runoff was minimal 

2.2 Sample handling 

2.2.1 Rock, tailings, and stream sediment 

Samples consisting of from 100 to 500 g of altered serpentinite and mine tailings were collected 

from exposures and waste rock piles in the Rathburn and Petray Mine pits. 125 mL jars were 

nearly filled with wet stream sediment. Rock and tailings samples were stored and shipped in 

zip-lock bags at ambient temperature. Wet sediment for total Hg (HgT) and methyl Hg (MMHg) 

analysis were stored on dry ice within approximately one hour after collection and kept frozen in 

the laboratory until shipped on dry or wet ice overnight to analytical laboratories, where upon 

receipt they were kept frozen until analysis. The temperature of samples arriving at the analytical 

facilities ranged from 1 to 4oC, which is within the limits specified in EPA Method 1631E. 

11 
Table 1. Altered serpentinite, calcine and sediment sample locations at open pits at the Rathburn and 

Petray Mines. 

[See fig. 2 for a map of these locations. Geographic coordinates are referenced to the NAD 27 CONUS] 

Location Latitude Longitude Description 

Rathburn calcine, sediment, and altered rock 

1 39.07507 -122.44825 Brick retort and soil

2 39.07607 -122.43822 Sand and silt in open cut

9 39.07677 -122.44838 Waste rock of altered serpentinite

10 39.07677 -122.44838 Altered serpentinite

Petray Open Pit sediment 

3 39.07677 -122.44492 Silt and clay sediment in middle pit

4 39.07677 -122.44825 Silt and clay sediment in middle pit

5 39.08605 -122.45140 Silt and fine sand from north-middle pit drainage

6 39.08578 -122.45098 Clay in north-middle pit

Petray ore and altered rock 

7 39.08178 -122.44933 Argillic alteration in shear zone

8 39.07680 -122.44490 Opal veins in silicified serpentinite

11 39.08608 -122.45155 Cinnabar on fractures in south trench

12 39.08876 -122.45208 North pit quartz-chalcedony vein in shear zone

13 39.08879 -122.45210 North pit opal veins in altered serpentinite

14 39.08606 -122.45140 Quartz-chalcedony vein in silica-carbonate rock alteration

15 39.08880 -122.45015 Waste rock east end, north open pit

R 39.0889 -122.4493 Medium-to-coarse sand below Petray north pit, north fork of 

tributary 1

S 39.0885 -122.4486 Pyroxenite sand in north fork of tributary 1 above confluence 

with north pit

15 39.1200 -122.4800 Waste rock channel, Petray north pit outlet

D 39.0949 -122.4424 Coarse sand sediment, north fork of tributary 1

V 38.0869 -122.4500 Sand below middle petray pit, north fork of tributary 1

12 
Table 2. Stream water and sediment sample locations, quantities, and dates. 

[See fig. 2 for a map of these locations] 

Location Latitude Longitude Water Sediment Dates Description 

Cain 

Spring 39.06765 -122.39883 1 1 12-20-04 Saline spring, Cain Valley 

A 39.09579 -122.43401

1 

1 

1 

1 

12-21-04 

12-23-04 

02-21-05 

Fine sand in bank deposit of tributary 1

B 39.09566 -122.44049

1 

1 

1 

12-21-04 

12-23-04 

Fine sand and silt in bed sediment of 

tributary 1

C 39.09408 -122.44180 2 2-21-05 Carbonate-cemented fine-to-coarse sand 

sediment, south fork of tributary 1

D 39.09498 -122.44233

1 

1 

1 

1 

12-23-04 

02/16/05 

05-27-05 

Fine-to-medium sand bed sediment, north 

fork of tributary 1

E 39.09568 -122.44143 1 05-31-05 Fine sand bed sediment, north fork of 

tributary 1

F 39.09572 -122.44038 2 05-31-05 Carbonate-silt sediment, south fork of 

tributary 1

G 39.09136 -122.43813 1 05-31-05 Fine-to-silt sand & clasts of Great Valley 

siltstone sediment, tributary 2

H 39.08657 -122.43434 1 05-31-05 Fine, black silt in dry creek bed with dunite 

clasts, north fork of tributary 3

I 39.08593 -122.43310 1 

1 

02-16-05 

05-31-05 

Fine-to-medium sand sediment, south fork 

of tributary 3

J 39.08239 -122.43082

1 

1 12/21/04 

2/16/05 

Medium sand sediment, tributary 4

K 39.09232 -122.44411 1 12/21/04 Carbonate-cemented clay and silt, south 

fork of tributary 1

M 39.08856 -122.44614 1 

1 

1 

1 

12-21-04 

12-23-04 

05-27-05 

Carbonate-coated clay and ferric 

(hydr)oxide precipitated from a saline 

spring

MB 39.08797 -122.44550 1 05-27-05 Upper part of terrace formed by deposition 

of carbonate minerals from spring 

N 39.09510 -122.42956

1 

1 

2 12-21-04 

05-27-05 

Medium-to-coarse sand sediment, tributary 

1

N 39.09502 -122.42932 1 0 12/14/06 

Lower reach of tributary 1, 5 m downstream 

from location N 

13 
Table 2 – continued 

O 39.09546 -122.44466 

1 

1 

02-16-05 

05-27-05 

Dead Shot Creek 

P 39.09502 -122.42932 1 1 05-28-05 Upper reach of tributary 1 below Petray 

north pit 

Q 38.92457 -122.32381 1 02-23-05 Spring along road cut above Bear Creek 

2.2.2 Water 

Spring and stream water samples were collected with new 60-mL sterile polypropylene syringes. 

Bulk water samples were subsampled for analysis of metal(loid)s and anions. Subsamples for 

metal(loid)s determinations were acidified to pH<2 with trace metal (Ultrex, J.T. Baker)-grade 

HNO3 and stored in acid-washed, high-density polyethylene (HDPE) bottles. Subsamples for 

anion and alkalinity measurements were filtered, stored in HDPE bottles, and chilled to 

approximately 4°C until analysis, in accordance with U.S. Geological Survey protocols for trace 

metals (http://pubs.water.usgs.gov/twri9A). Water samples were filtered in the field for analysis 

of anions, alkalinity, and ICP-MS and ICP-AES analysis. Pore fluids were filtered in the lab with 

0.45 μm syringe filters within a few days of collection.

Water parameters including pH, conductivity, dissolved oxygen, and temperature were measured 

in the field with a Hydrolab multimeter, either by placing the probe directly into the flowing 

stream water (fig. 14) or by filling a container and placing it over the probe. 

Samples for HgT and MMHg analyses were collected in trace-metal-free-certified 1-L 

borosilicate bottles (Nalgene ICHEM) with minimal headspace and Teflon3-lined caps. Samples 

were kept on ice until shipped. Samples were shipped on dry ice or blue ice packs, arriving the 

next morning at the analytical facilities at temperatures ranging from 1 to 9o C. 

Some samples arrived at temperatures exceeding 4oC, which is specified by EPA Method 1631E 

to minimize biologically induced phase changes and MMHg degradation. 

Samples collected for dissolved organic carbon (DOC) analysis were filtered in the field with 

0.45 μm filters into amber borosilicate vials with Teflon-lined caps and put on ice. Samples were 

refrigerated at 4o C for approximately five days, shipped overnight to the analytical lab, and 

analyzed within a few days. 

2.3 Analytical methods 

In addition to documenting methods, the following sections mention sample preparation issues 

that appreciably affect the interpretation of the results reported in Section 3. 

2.3.1 Sediment and tailings 

Multi-element analyses for all tailings (material processed in a retort, also referred to as 

calcines), waste rock (material moved from open pits and adits but not processed), and dry 

sediments were performed in the laboratories of ALS Chemex (Reno, NV) or SGS Minerals 

Services (Elko, Nevada; formerly X-ray Assay Laboratories (XRAL)). Bulk samples were 

 

3 Any use of trade names is for descriptive purposes only and does not imply endorsement by the U.S. government.

14 
ground in a zirconia ring mill and subjected to a near-total four-acid digestion. Major elements in 

digests were determined by inductively coupled plasma-atomic emission spectroscopy (ICPAES). Trace elements other than Hg were determined by inductively coupled plasma–mass 

spectrometry (ICP–MS). Hg was determined by cold vapor atomic absorption spectroscopy 

(CVAAS) following methods similar to those described by Crock (1996) and O’Leary and others 

(1996). 

Hg analyses of altered serpentinite, tailings, and waste rock, and wet sediment were performed 

by Frontier Geosciences Inc. and Brooks Rand LLC (Seattle, WA). Solids were not homogenized 

prior to subsampling the approximately one gram of sediment from the bulk (on the order of 

100g) sample taken in the field. Nor did either laboratory grind subsamples prior to digestion. 

This approach unnecessarily introduced an additional source of inhomogeneity, or a possible 

“nugget” effect because some samples contained abundant fine grained HgS. We recommend 

that future investigators discuss sample preparation measures in detail and, if homogenization 

cannot be provided, homogenize subsamples themselves if particulate HgS is present in the 

sample. 

At Frontier Geosciences, subsamples for total Hg (HgT) analysis were digested at room 

temperature with aqua regia (10 parts HCl and 3 parts nitric acid) (Bloom and others, 2003; 

Fernandez-Martinez and Rucandio, 2005). At Brooks Rand, for HgT, approximately one gram of 

sediment was digested with 7:3 conc. nitric:sulfuric acids and heated to 100°C for one hour, 

followed by refluxing at 150°C for two hours in a glass vial with Teflon-lined lids. Bromine 

monochloride BrCl was added to the digest after it had cooled to room temperature, diluted with 

deionized water such that the concentration was within instrumental range), and followed by 

stannous chloride reduction, single-stage gold amalgamation, and cold vapor atomic fluorescence 

spectroscopy (CVAFS) (U.S. EPA Method 1631E). The acid digest procedure should recover 

elemental, adsorbed, and amalgamated Hg, and Hg sulfide (HgS(s); cinnabar and metacinnabar), 

but does not appreciably dissolve silicate minerals. While previous studies suggest that Hg is 

sorbed to silicate minerals that comprise some if not most of the solids sampled here (excepting 

carbonate-rich concentrates), only minor fractions of Hg have been found to be within the 

aluminosilicate mineral structure (Sladek and Gustin, 2003; Lowry and others, 2004; Slowey and 

others, 2005). Detection limits for HgT in solids by this method ranged from 0.03 to 0.3 ng/g 

based on three standard deviations of method blank recoveries. 

MMHg in sediment was determined by acid bromide-methyl chloride extraction, followed by 

aqueous phase ethylation, trapping (Carbotrap and Tenex; Scientific Instrument Services, Inc.), 

isothermal gas chromatographic separation, and CVAFS detection modification of EPA Draft 

Method 1630 (Horvat and others, 1993b). Results are reported on a wet-weight basis along with 

gravimetrically measured water contents. Detection limits for MMHg in solids ranged from 

0.008 to 0.010 ng/g based on three standard deviations of method blank recoveries. Recoveries 

from certified reference materials at both analytical facilities did not indicate that they had 

problems analyzing certified reference materials. Two matrix spike recoveries from the Rathburn 

soil/retort brick sample 1 were 87% and 104% of 62,000 ng HgT/g and 95% and 103% of 5.0 ng 

MMH/g. Analytical precision based on these matrix spike duplicate analyses was 1σ = 0.06 ng 

MMHg/g, or 11% of the mean concentration, and 640 ng HgT/g, or 14% of the mean 

concentration. Sediment subsamples were not tested for analytically induced formation of 

MMHg due to potentially high organic matter content (Bloom and others, 1997).

15 
2.3.2 Water 

The acid-neutralizing capacity (ANC) of samples collected in 2005 was measured at room 

temperature on November 14, 2006 after thawing samples frozen in polypropylene bottles. 

Samples for ANC were collected in December 2006 in polypropylene bottles, transported on wet 

ice to the laboratory, refrigerated for one week, and then titrated. A titration system equipped 

with a 1 μL-resolution burette delivered 1.6 N sulfuric acid at 10 to 100 μL/min to stirred, open 

30 to 50 mL samples. Titration data points were numerically interpolated. Potential equivalence 

points (pH at which a buffering constituent(s) is titrated) were identified from peaks in the first 

derivatives of interpolated titration curves, known as the inflection point titration (IPT) method 

(USEPA 310.1) (Rounds, 2006). Waters containing saline spring water likely contain buffering 

agents other than carbonate (for example, organic acids and humic substances). 

Sulfate, chloride, nitrate, and fluoride were determined by ion chromatography (Fishman and 

Friedman, 1989). Cations were analyzed by inductively coupled plasma atomic absorption 

spectrometry (ICP–AES) and mass spectrometry (ICP–MS). Duplicate water samples, blank 

samples, and USGS Water Resource Division standard reference waters were analyzed with the 

data set and yielded acceptable results. 

Total Hg (HgT) was measured in samples collected in 2004 and 2005 by USEPA Method 1631 at 

Brooks Rand and Frontier Geosciences (Seattle, WA). Water samples collected in December 

2006 were sent to Studio Geochimica (Seattle, WA) for HgT analysis by USEPA Method 1631. 

MMHg was analyzed at all facilities using aqueous phase ethylation, GC separation, isothermal 

decomposition, and CVAFS detection (Horvat and others, 1993a). Analytical detection limits for 

HgT and MMHg in water ranged from 0.1 to 0.2 ng/L and 0.01 to 0.02 ng/L, respectively, based 

on three standard deviations of blank recoveries. The field reagent blank recovery analogous to 

HgT sample preparation (0.5 percent v/v BrCl in deionized water) was 3.59 ng/L, indicating that 

the BrCl reagent used to preserve HgT samples in the field contained measurable Hg. As reported 

in section 3, this level of Hg ranged from 1 to 60 percent of concentrations recovered from water 

samples (23 percent on average from 16 recoveries of HgT and HgD combined). Although this 

contamination did not affect conclusions provided in this report, it should be considered in future 

interpretations. 

Concentrations of dissolved iron and dissolved organic matter (DOM) in some spring water 

samples collected in December 2006 resulted in precipitation of ferric hydroxide and coagulation 

of humic substances (Myneni, 1999) upon addition of acidic BrCl reagent in the field. Both ferric 

hydroxide and coagulated humic substances can sorb Hg(II) and therefore may have partially 

impeded full recovery of Hg from the bulk samples provided for analysis. Two tests were 

performed to assess the potential for this problem. Two samples were analyzed at several 

dilutions. If a Hg-exchangable interferent (for example, a sorbent) is present, the dilutioncorrected measured concentration of Hg would be observed to increase as the sample is diluted. 

This was not observed, suggesting that interference due to sorption did not occur. The second 

test involved adding incrementally higher levels of BrCl (up to 5% v/v) to one water sample to 

determine if some compound (for example, organic or reduced sulfur) was preventing Hg(II) 

from being reduced to Hg(0) to be purged and quantified. In no case did the sample tested release 

additional Hg(0) after incrementally stronger BrCl digestion and SnCl2 reduction. This further 

suggests that interference due to sorption did not occur. 

16 
Because of the high concentration of dissolved organic matter (DOM) in some of the samples 

collected in December 2006, Studio Geochimica investigated the potential for potential positive 

artifact formation of MMHg during the aqueous distillation step (Bloom and others, 1997). This 

check was not performed on the 2004 or 2005 samples but all of these samples were surface 

waters and only two were spring waters with high DOM. 

Total dissolved organic carbon (DOC) and ultraviolet absorbance at 254 nm (UV254) was 

measured by USEPA Method 415.3 with modifications according to Aiken (1992). UV254 is 

normalized by DOC concentration to yield an operational parameter called the specific 

ultraviolet absorbance (SUVA). SUVA is an indirect characterization of the degree of 

aromaticity of dissolved organic carbon compounds (Weishaar and others, 2003). A 

complementary characterization of DOC was performed by measuring UV-spectral fluorescence 

over a range of UV wavelength excitation energies. The fluorescence intensities were compiled 

into three-dimensional excitation-emission matrices (EEMs), from which a two-dimensional 

emission spectrum and a one-dimensional fluorescence index (FI) were obtained at an excitation 

wavelength of 370 nm (McKnight and others, 2001). The FI was computed by dividing the 

emission intensity at 470 nm (I470) by that at 520 nm (I520) (both measured at 370 nm excitation). 

3. Results and discussion 

Given the scale and complexity of the Rathburn and Petray Mines-Bear Valley system and the 

limitations of the current database, this section summarizes findings while providing preliminary 

interpretation. After summarizing the Hg results for tailings and altered rock, the results for 

stream water, sediment, and saline ground water seepage along the Bear Fault are presented. 

These discussions develop preliminary source-receptor relationships throughout the region and 

begin to identify Hg transport processes. Since the brevity of this study leaves appreciable data 

gaps and uncertainty, recommendations for a future hypothesis-driven study are provided in 

section 4 on the basis of considerations presented in section 1.4. 

17 
Table 3. Elemental composition of tailings, waste rock, and sediment at the Rathburn and Petray Mine sites. 

18 
Table 3 - continued 

19 
20 

3.1 Tailings and dry sediment at the Rathburn and Petray Mines 

The concentration of Hg in waste materials at the Rathburn Mine varies depending on material 

type. Ores were processed at the Rathburn Mine in a brick retort, and a small amount of calcines 

are present near the remains of the retort. The calcines sampled in 2001 contain 1020 ppm Hg, 

compared to 470 ppm measured in similar material collected by Churchill (Churchill and 

Clinkenbeard, 2002) (table 3). Waste rock derived from open cuts developed to mine Hg ore 

from altered serpentinite contains considerably less Hg (39 ppm), as does the exposed altered 

serpentinite (12 ppm). Waste material and sediment has accumulated in several of these open 

cuts. Sediment in one of these disturbed areas contains 36 ppm of Hg (table 3). While calcines 

contain much higher Hg concentrations than waste rock and serpentinite, they comprise a minor 

fraction of the mass of material exposed at the Rathburn Mine. 

Unlike the Rathburn Mine, Petray Mine Hg ores were processed offsite in a rotary furnace at the 

Abbott Mine during 1971 with only 10-100 flasks of Hg produced, according to limited 

production records (Churchill and Clinkenbeard, 2002). Visible cinnabar is present on fractures 

in the south and central pits, with the largest area of visible cinnabar mineralization occurring in 

the northeast part of the central pit where 2490 ppm of Hg was measured (location 11). Other 

areas of altered rock in the south and central pit contained considerably less Hg (2.5 to 53 ppm). 

The north Petray pit was developed to expose several quartz-chalcedony veins that trend 

northwest. No cinnabar was observed, and 180 ppm Hg was measured in a visually 

representative sample. The Hg concentration in adjacent altered serpentinite was low (4.6 ppm) 

and similar to concentrations measured in serpentinite in the south and central pits. Due to its 

abundance, waste rock consisting of altered serpentinite is the primary Hg-enriched material of 

environmental interest at the Petray Mine. The amount of surface disturbance is relatively large 

given the small Hg production when compared to other Hg mines in the Coast Ranges of 

California. 

Some, but not all, of the large waste rock piles that exist at the Petray Mine’s south pit have 

apparent hydrological transport pathways to adjacent drainage. A large waste rock pile northeast 

of the south pit (previously called the “central pit” by Churchill and others 2002) contains 80 to 

170 ppm Hg. Erosion of this waste rock contributes Hg-enriched sediment to Tributary 3 (fig. 2), 

on the basis of 90 ppm of Hg measured in sediment immediately downstream of the pile 

(Churchill and Clinkenbeard, 2002). In contrast, altered rock eroded from pit walls and waste 

rock from adjacent piles in the south pit are apparently trapped in the pit with no apparent 

hydrological outlet. Sediment in this vicinity contains elevated Hg (120 to 250 ppm), on the basis 

of measurements of material collected from locations 3 and 4 (fig. 2). Again, this material will 

not likely move off-site as long as the area remains undisturbed. 

High concentrations of Hg in sediments in the north fork of tributary 1 result from the release of 

Hg-enriched sediment from two of three open pits at the Petray Mine. The Petray central pit 

(called the “south pit” by Churchill and others 2002) contains several small waste rock piles with 

Hg concentrations ranging from 170 to 1070 ppm. Hg-enriched sediment (42 ppm, location 6) 

accumulates in the western part of the pit, most likely after eroding from altered rock on the 

walls of the pit. The northeast end of the central pit does have an outlet leading to the south fork 

of tributary 1, and Hg-enriched sediment has been measured there (160 ppm, location 5). A large 

waste rock pile located at the east end of the Petray north pit occurs on a very steep slope, has 

been partially eroded, and is likely a source of Hg-enriched sediment to the north fork of 
tributary 1 (fig. 2). A sample collected from this pile contained 178 ppm Hg, and sediment 

downstream of the point where drainage from the north pit enters tributary 1 contained a Hg 

concentration higher than that above the confluence (410 ppm at location R, versus 320 ppm at 

location S; table 4). Thus, high concentrations of Hg in sediments in the north fork of tributary 1 

likely result from the release of Hg-enriched sediment from both the central and north pits. 

Very high concentrations of Hg (46-355 ppm) in sediment persist in the active channel of the 

north fork of tributary 1 downstream from the central and north Petray Mine pits. However, Hg 

concentrations vary considerably and do not consistently decay from the source. Where the 

North fork of tributary 1 enters Bear Valley, the Hg concentration in sediment is variable but 

comparable to upstream locations closer to the North pit, on the basis of limited measurements: 

357, 250, and 136 ppm (location D on Dec. 2004, Feb. 2005, and May 2005, respectively; table 

4) and 355 (location E, Dec. 2004). Cinnabar was panned at both of these locations. Pan 

concentrate from this site was analyzed by Hg LIII edge extended X-ray absorption fine structure 

(EXAFS) spectroscopy and found to contain equal proportions of cinnabar and metacinnabar 

(data not shown), suggesting that these are the main Hg phases in sediments. 

If relatively coarse mineral particles account for the Hg present in sediment, measurements of 

total Hg should be expected to be variable (because of the nugget effect) and provide only a 

limited basis on which to derive reliable Hg loading estimates for the north fork of tributary 1 to 

Bear Creek. Further downstream on tributary 1, after inflow from the south fork, the sediment 

Hg concentration decreases (52 and 75 ppm at location A, 60 ppm, location N), suggesting that 

sediment transported from the south fork of tributary 1 and Dead Shot Creek dilutes Hg 

transported from the north fork of tributary 1. At location A, conglomerate exposed in the walls 

of the creek contains 0.86 ppm Hg, indicating that premining sediment has a much lower 

concentration than that present in the active channel. Concentration of Hg in sediment from 

tributaries that do not drain the mine area (that is, tributaries 2, north fork of 3, and 4), have a 

lower Hg concentration (0.16-0.23 ppm Hg) than the premining sediment (0.86 ppm Hg) 

exposed in tributary 1, indicating that natural erosion of the Petray deposit contributed Hg 

enriched sediment to this tributary. 

Sediment in the Bear Creek tributaries in Bear Valley is partially cemented with carbonate 

minerals. Spring water nearly or fully saturated with calcium carbonate enters the south fork of 

tributary 1 downstream of the Petray south pit. This water is at times supersaturated with respect 

to calcium or magnesium carbonate, based on observed deposition of carbonate minerals on the 

creek bed and cementation of sediment in the active stream channel (fig. 23). Cementation of 

particles should limit the amount of Hg-enriched sediment that is transported from the upper part 

of this tributary to Bear Creek, as discussed further in the following section. The sediment in this 

tributary contains relatively low concentrations of Hg, ranging from 0.9 to 5 ppm. 

3.2 Surface and saline ground water seepage 

In addition to the Hg content of sampled waters, additional parameters are discussed below to 

evaluate which components can reliably indicate the provenance of water observed in the Bear 

Creek tributaries adjacent to the Rathburn and Petray Mines. This combined approach estimates 

the proportions of two presumptive end-member sources—meteoric water and saline ground 

water derived from connate fluids in sedimentary rocks in the Great Valley Sequence—and their 

Hg content, indicating the importance of each to the Hg contamination of Bear Creek. The 

21 
following paragraphs focus on the distinct properties of each end-member on the basis of 

analyses of water collected at times when we assume the samples were dominated by one endmember. For example, surface water collected in February 2005 during a rain storm is less 

dominated by saline ground water seepage than in May 2005 under dry-season conditions. 

Bulk water quality parameters, such as pH and specific conductivity (SC), reflect the influence of 

saline spring and ground water in stream channels draining the Petray Mine and within Bear 

Valley. Specific conductivities vary widely (standard deviation (s) = 1.8 mS cm-1 for 2004-2005 

stream waters and s = 6 mS cm-1 for saline ground water seepage collected in December 2006) 

and are not clearly diagnostic of whether the water in the Bear Creek tributaries is predominantly 

meteoric versus ground water (t test P = 0.09, comparing the means of each dataset). For 

example, SC is similar in waters sampled during periods of high and low meteoric input, as 

determined on tributary 1 water collected in February and May 2005. This is likely due to rapid 

dissolution of carbonate minerals in tributary stream beds by acidic meteoric water during the 

wet season. The exceptionally high conductivities measured along the central part of the Bear 

Fault (for example, 9.4 and 19 mS cm-1; table 6) reflect the significant component of saline 

ground water in these springs. 

Alkalinity is relatively high in both spring and surface waters because of contributions from 

saline ground water and interaction of meteoric water with serpentinite and ultramafic country 

rock. Alkaline pH from 8.0 and higher (table 6) suggests aqueous equilibrium with calcium 

carbonate in the stream channels (Stumm and Morgan, 1996). The alkalinities of all sampled 

waters varied greatly, from 2.8 meq L-1 at location S (presumably less dominated by ground 

water) to 29 meq L-1 in water collected at a saline cold spring that is actively depositing travertine 

and is estimated to consist of about 20% saline connate ground water (location M, fig. 2). Most 

waters in the Bear Valley tributaries are highly alkaline (table 6 and fig. 5). However, the eight 

saline ground-water seepage samples collected along the Bear Fault had alkalinities (9.7 to 26.3 

meq L-1; average 15.4 meq L-1, s = 5.7 meq L-1) that were significantly higher, by 80 percent on 

average, than nine stream surface waters sampled in 2004-2005 (averaging 8.6 meq L-1

, s = 3.3 

meq L-1; t test P = 0.013)4. This reflects the relatively larger component of saline ground water in 

these springs. On the basis of the topology of the acid-titration curves (fig. 5), bicarbonate was 

most likely the dominant buffering agent, although organic matter may also buffer these waters. 

 

4

 The saline spring (location M) alkalinity datum was excluded from this statistical analysis. 

22 
Table 4. Hg and methyl Hg in surface water, sediment, and porewater and sulfide in pore water. 

[Italicized sediment Hg data are results from larger, homogenized samples] 

Location Date Hg in water 

(ng/L)1 

Methyl Hg 

in water2 Hg in sediment3,6 Methyl Hg in sediment4 

 

Unfiltered 

Filtered 

(<0.45 

μm) 

Unfiltered 

wt-% 

solids

μg/gwet μg/gdry1 

ng/gwet ng/g-dry5 

Cain 12/20/04 16 5.4 0.12 0.45 0.07 2.5 

Spring 5/23/05 - - - 0.54 0.07 0.82 

0.58 44 76 0.82 

A 12/21/04 460 - - - 52 

- 0.9 

12/21/04 14 5.8 <0.02 0.52 23.5 45 0.6 

B 

5/23/05 - - - 0.65 46 0.08 

- 5.0 - C 12/21/04 - - - - 1.4 - 

12/21/04 860 - - - 360

D 2/21/05 50 11 -

5/31/05 75 9.0 - 0.73 188 5.8

E 12/21/04 - - - - 360

12/21/04 0.21 7.5 36 68

F 

5/23/05 - - - 0.28 10 57

G 12/21/04 - - - - 0.23 -

H 12/21/04 - - - - 0.19 -

12/21/04 - - - - 38 - I 

2/21/05 122 9.9 - - - -

12/21/04 - 0.16 - J 

2/21/05 12 9.5 - - - -

K 12/21/04 - - - - 0.86 -

12/21/04 20 - - 0.26 0.6 2.3 0.21

L 

12/21/04 - - - 0.68 52 76 0.06

12/21/04 2.0 1.1 <0.03 0.16 0.02 0.3 2.0

M 

5/23,27/05 - - - 0.21 0.04 -

MB 5/27/05 20 1.1 - 0.23 1.7 6.0

23 
Table 4 – continued 

12/21/04 690 440 0.05 - 60 - N 

5/27/05 588 24 - - - -

2/21/05 7.8 4.4 - - - - O 

5/31/05 3.9 0.9 - 0.8 0.09 <0.01

P 5/26/05 110 15 - 0.75 250 337 0.27

Q 2/23/05 1.1 0.5 - - - -

R 5/26/05 - - - 408 

S 5/26/05 - - - 319 

V 5/23/05 - - - 139 

06RP1 102 110 0.06

BF1 37 32 0.60

BF7 40 29 0.29

BF8 38 32 0.33

BF2 6.9 6.6 <0.04

06RP8 7.6 5.9 <0.04

06RP9 

12/13-14 

/06 

252 - 0.09 

Pore water Sulfide, μmol/L 

BF1 - 9.03 0.24 2.4 ± 0.17

BF2 - 14.3 0.54 3.5 ± 1.1

BF7 

12/13-

14/06 

- 18.9 3.4 4.4 ± 1.7

1-4Analytical detection limits based on 3s of reagent blank recoveries were 1 0.2 ng/L, 2

0.04 ng/L, 30.12 

ng/g, and 4

0.02 ng/g. 5

Concentrations on a dry weight basis were computed by dividing the ng/g-wet sediment by the 

weight-% solids of the sediment. 6

Matrix spike recovery was less than unspiked recovery, indicating substantial heterogeneity in the 

distribution of Hg within the sediment sample. The true bulk concentration of this and the other sediment 

samples may be substantially different than these reported values. 

7

Error intervals for sulfide are 1σ of replicate sample analyses.

24 
3

4

5

6

7

8

9

0 0.2 0.4 0.6 0.8 1

Location D

O

I

J

S

R

D

M

NB

pH

mL 1.6 N sulfuric acid

a

 

3

4

5

6

7

8

9

0 0.1 0.2 0.3 0.4 0.5 0.6

06RP1

06RP3D

06RP5

06RP6

06RP7

06RP8

RP10

06RP11

pH

mL

b

Figure 5. Acid titration curves for filtered (<0.45 μm) Rathburn and Petray Mine drainage sampled in May 

2005 (a) and Bear Valley saline spring-influenced surface water sampled in December 2006 (b). 

If the waters are simply binary mixtures of meteoric water and saline ground water, all 

conservatively transported ions should show a linear mixing line when plotted against Cl. A plot 

of Na verses Cl (fig. 6) demonstrates that surface and spring waters plot along a two-endmember linear mixing line with springs having higher concentrations of Na and Cl than surface 

waters. The most saline spring water with the highest Na concentration is from a CO2 spring (fig. 

20, sample 06RP06 in table 6) located in the central part of the Bear Fault. This water likely 

represents the saline ground water source end member. The concentrations of Cl and Na in this 

water are comparable to saline geothermal spring waters in the Sulphur Creek to the south of the 

study area. 

In comparison to all other waters in the study area, this water also has the highest concentration 

of K, Rb, Li, Se, Sr, W, and nitrate, sulfate, and DOC. Plots of these cations against Cl define a 

linear mixing line similar to that for Na and Cl and indicate that these elements behave 

conservatively. In contrast, because of carbonate mineral precipitation in the tributaries and at 

springs, Ca and Mg do not plot on a linear mixing line. 

25 
Figure 6. Correlation between log10 transforms of Na and Cl in filtered (<0.45 μm) spring, pore, and 

surface water in streams draining the Petray Mine and Bear Valley. 

[The ANOVA statistics shown in the figure suggest that a correlation this large (R2

 = 0.97) could happen 

by chance alone only 0.07 percent of the time. The dashed line represents the mixing line between the 

most saline spring water and background meteoric surface water in tributary 4. The percent (%) connate 

Na-Cl ground water in each sample is based on the Cl concentration present in the water sample and that 

in the two end members of the mixing line]

The relative percentage of meteoric and saline ground water can be determined for various spring 

and surface waters on the basis of the computed binary mixing line (fig. 6) where the most saline 

spring water is defined as the pure ground water end member and the pure meteoric water is 

defined by surface water from a background site in tributary 4 sampled during a storm event 

(sample J, table 6). The percentage of ground water in each water sample is: 

 % ground water = ClMW −Clmeas

ClMW −ClGW

⎡

⎣ ⎢ 

⎤

⎦ ⎥ •100 (equation 1)

where the subscripts MW and GW denote the chloride concentration in the end member meteoric 

water and saline ground water, and meas is the concentration in a water sample. Both spring and 

surface waters contain less than 50% saline ground water (fig. 6, table 5). The highest component 

of saline ground water is present in spring waters along the central part of the Bear Fault. Pore 

waters and springs sampled at either end of the fault zone are predominantly meteoric water. 

26 
Effluent from springs associated with the travertine terrace in the head waters of the south fork of 

tributary 1 causes the relatively high component of saline ground water in this tributary at site B. 

A simple mixing of saline ground water and meteoric water does not explain the observed Hg 

concentrations in spring or surface waters. The concentrations of Hg in both filtered and 

unfiltered waters are not correlated with chloride concentration or any other major or minor 

element. Processes that control mercury concentration in surface and spring waters are not 

conservative and may include precipitation and dissolution of carbonate, dissolution of HgS by 

DOC, and seasonal changes in surface water flow. These processes are discussed in detail below. 

The highest total Hg concentration (855 ng/L) measured under dry season low flow conditions 

occurs in surface water in the north fork of tributary 1 (location D). The waters in this tributary 

are predominantly meteoric, less than 1.5% saline ground water, in both the dry and wet season. 

This site has the highest concentration of Hg in sediment (350 ppm) and HgS can be panned 

from the sediment. 

Spring waters that emanate from the central part of the Bear Fault also have elevated 

concentrations of Hg with the highest total Hg (102 to 690 ng/L) and Hg in filtered water (24-

435 ng/L) occurring where tributary 1 crosses the fault (locations A and N, and samples 06RP 1 

and 9, table 5). Spring waters located along the fault contain a significant but highly variable 

proportion of saline ground water, from 13 to 100 % (table 5), and the Hg concentration in the 

spring waters is not correlated to the proportion of saline ground water present. Pore fluids 

sampled from the north and south end of the Bear Fault also have elevated concentrations of Hg 

even though these waters are predominantly meteoric. 

Seasonal changes in flow in some cases increase Hg concentration in surface waters (tributary 3) 

and in other cases decrease it (north fork tributary 1).Sampling of one storm event was carried 

out in the middle of the 2005 wet season (February). Elevated concentrations of total Hg (122 

ng/L) were measured in tributary 3 that receives runoff from the Petray Mine. The high 

concentrations of Hg present in the north fork of tributary 1 were considerably lower during the 

storm event, 49 ng/L as compared to 855 ng/L during low flow conditions. At the background 

site in tributary 4 (site J) low concentrations of total Hg, 12.4 ng/L,and Hg in filtered water, 9.5 

ng/L, were present during the storm sampling. Somewhat lower concentrations of total Hg (7.8 

ng/L total Hg and 4.4 ng/L in filtered fraction) occur in Dead Shot Creek that is not impacted by 

mining. 

27 
Table 5. Predicted proportions of saline ground water in water samples based on Cl concentration 

in spring, pore, and surface waters. 

[Weather conditions and mercury concentration are listed] 

Weather Cl- % Saline Hg in water Location Site Description 

Condition (mg/L) Ground water (ng/L) 

Springs along Bear Fault Unfiltered 

Filtered 

(<0.45 μm)

06RP1 At tributary #1 Dry season 1445 23.8 102.1 109.9 

A At tributary #1 Dry season 1882 31.0 458 

N At tributary #1 Dry season 1600 26.3 690 435 

 At tributary #1 Dry season 811 13.3 588 24 

06RP6 CO2 Spring Dry season 6065 100.0 38.08 32.43 

06RP7 Carbonate deposition Dry season 2757 45.4 6.89 6.57 

06RP8 No carbonate deposition Dry season 2210 36.4 7.60 5.93 

Springs with travertine terraces 

M Spring terrace Dry season 1184 19.5 

M Spring terrace Dry season 1202 19.7 1.97 1.05 

Cain Cain Spring 

 Spring Cain Spring Dry season 543 8.9 15.9 5.44 

Tributaries 

D North fork, tributary 1 Rain storm 46 0.7 49.3 11.4 

D North fork, tributary 1 Dry season 98 1.5 855 

D North fork, tributary 1 Dry season 66 1.0 75.3 9.04 

MB South fork, tributary 1 Dry season 13 0.1 19 1.06 

B South fork, tributary 1 Dry season 820 13.4 14.1 5.81 

L South fork, tributary 1 Dry season 8 0.0 19.9 

I Tributary 3 Rain storm 7.3 0.0 122 9.93 

J Tributary 4 Rain storm 5.2 0.0 12.4 9.46 

O Dead Shot Creek Dry season 864 14.2 3.94 0.85 

O Dead Shot Creek Rain storm 604 9.9 7.78 4.40 

Pore fluid along Bear Fault 

06RP3 North end, pore fluid Dry season 113 1.8 37.06 31.83 

06RP5 South end, pore fluid Dry season 29 0.4 40.00 29.21 

28 
Dissolved organic carbon (DOC) may qualitatively indicate the proportion of meteoric surface 

water versus ground water. The pure saline ground water end member has the highest DOC. 

Three other springs along the Bear Fault with lower proportions of saline ground water have 

lower DOC, with log DOC correlated to the log percent saline ground water present (R2

 = 0.92). 

In contrast, pore fluids sampled near the north and south end of the fault consist of meteoric 

waters with relatively high DOC indicating a terrestrial source. In addition, DOC is 

biogeochemically relevant to Hg and its methylation. DOC is abundant in saline ground water 

seepage and consists of a complex mixture of aliphatic acids and humic substances. The yellow 

color of most of the seepage collected along the Bear Fault is characteristic of aquatic fulvic 

acid, but we presume, with reservation (see below), that the origin of the organic matter is from 

connate fluids that have leached both marine and terrestrial organic matter from sedimentary 

rocks of the Great Valley Sequence. DOC analyses of filtered (0.45 μm) spring and pore waters 

collected along the Bear Fault in December 2006 found DOC concentrations ranging from 2.4 to 

30 mg L-1 carbon that, on average (21 mg L-1

, n = 6) were elevated relative to most fresh waters 

(table 7). Specific ultraviolet absorbance at 254 nm (SUVA254) values range from 2.1 to 5 L·g C-1

m-1 (averaging 3.8 L·mg C-1

 m-1, n = 6), which are moderate to high compared to a variety of 

marine and freshwater determinations. According to a model developed by Weishaar and others 

(2003), these SUVA254 values suggest that 17 to 36 percent of the organic carbon in these ground 

water seepage samples belong to aromatic molecules or moieties or, more specifically, sp2-

hybridized carbon. 

Aromatic carbon is important to Hg transport because it can dissolve HgS(s) (cinnabar and 

metacinnabar) and chemically stabilize dissolved aqueous Hg(II) complexes (Benoit and others, 

2001; Qian and others, 2002; Waples and others, 2005; Drott and others, 2007) (fig. 4). The 

percentage of aromatic carbon derived from SUVA254 should be considered as estimates and 

subject to correction based on pH and metal ion concentrations (especially transition metals), 

which can affect UV254 absorbance regardless of aromatic carbon content. Comparisons would 

only be valid when matrix similarity with respect to such constituents can be demonstrated. 

Nevertheless, the DOC analysis elucidates that ground water seeping along the Bear Fault 

contains appreciable amounts of organic carbon that could be capable of dissolving Hg(II), 

including cinnabar transported from the Rathburn and Petray Mines. In December 2006, HgT in 

water sampled from tributary 1 at a seep was less than half that measured tens of meters 

downstream in a larger pool (fig. 23) (102 ng L-1 and 252 ng L-1, respectively; table 5). The Hg 

concentrations measured in filtered and unfiltered seepage sampled at the upstream location were 

similar, suggesting that the ground water seepage contained predominantly dissolved Hg. The 

increased HgT (HgF is not available) at the downstream location further suggests that DOC in the 

seepage enhanced dissolution of Hg in the tributary 1 channel, possibly from cinnabar 

transported from the Petray Mine. 

Another useful parameter and DOC characteristic is ultraviolet (UV) fluorescence upon 

excitation with UV radiation. Figure 7a shows a representative matrix of fluorescence 

(sometimes referred to as emission) intensity as a function of excitation wavelength, called the 

emission-excitation matrix (EEM). Figure 7b shows a two-dimensional view of this and the other 

measured EEMs at an excitation wavelength of 370 nm. Further dimensional reduction yields 

fluorescence indices (FI), obtained by dividing the emitted intensity at two wavelengths 

(typically near the maximum and half-maximum intensity) for a given excitation energy (see 

guide lines in fig. 7b; FIs are reported in table 7). The FI takes into account the relative intensity 

and width of the fluorescence spectra, and so can be compared among samples. Interpreting FIs 

29 
in our study relies on previous studies that quantified carbon speciation using 13C nuclear 

magnetic resonance. However, we were unable to find a report of FI for connate fluid, and so this 

characterization is currently preliminary. 

While we have tentatively maintained that DOC in the saline ground-water-dominated water 

sampled in December 2006 came from connate fluid (that is, thermally degraded organic matter 

from Cretaceous Great Valley sedimentary rocks), the SUVA254 and FI results together question 

whether an appreciable fraction of DOC does not derive instead from terrestrial plant matter 

excreted from livestock that graze in parts of Bear Valley. The FIs of the December 2006 

sampled Bear Fault waters, ranging from 1.24 to 1.45 (table 7), are similar to waters containing 

DOC primarily sourced from degraded terrestrial organic matter, as opposed to microbially 

generated DOC, which typically has a higher FI (McKnight and others, 2001). A lower FI, 

coupled with a higher SUVA254 value,further suggests that DOC is proportionally more aromatic. 

This coupling is noteworthy given that FI is significantly correlated to SUVA254 (R2

 = 0.92, P = 

0.008) for the Bear Fault ground water seepage sampled (fig. 8). 

In December 2006, we did not find evidence of grazing in the more southern Bear Fault 

sampling locations (06RP5 and 6), but grazing had recently occurred near the north locations 

(06RP1, 3, 7, and 8). As shown in figure 2, a fence restricts grazing in the Rathburn/Petray 

portion of Bear Valley. Pore fluids sampled in an area of apparently recent grazing, location 

06RP3, had the highest SUVA254 and lowest FI, suggesting a larger fraction of aromatic carbon 

consistent with terrestrial organic input. Because the pore fluid is meteoric water, appreciable 

fractions of aromatic carbon from connate water could not explain this result. However, it would 

be useful to have further data to characterize the typical FI and SUVA254 values of deep ground 

water and various surface and pore waters in Bear Valley. It is also important to know the 

sensitivity of SUVA254 and FI to short-term land-use change (if samples were collected frequently 

enough), and to determine if end-member waters have distinct SUVA254 and FI signatures and if 

mixing of ground and surface waters significantly changes these parameters. The DOC results 

from December 2006 sampling are too few to be definitive with respect to these questions. The 

relatively high DOC and aromatic carbon in pore fluids can be attributed to terrestrial organic 

matter inputs linked to grazing on the basis of SUVA254 and FI. This realization emphasizes the 

importance of (1) coordination with land owners to better understand short-term land-use 

practices and (2) closer monitoring of meteoric water inputs to the system. With constraints 

applied by knowledge of land use and hydrology, continued measurement of DOC, SUVA254, and 

FI are recommended to help understand water dynamics in the Bear Valley system. 

30 
Table 6. Temperature, pH, alkalinity, and concentrations of selected anions. 

[mS/cm = millisiemens per centimeter, meq/L = milliequivalents of charge per liter, mmol/L = 

millimoles per liter, μmol/L = micromoles per liter] 

Sample Date T (°C) pH 

Specific 

conductivity 

(mS/cm) 

Acidneutralizing 

capacity 

(meq/L) 

Cl- 

(mmol/L) 

SO4

2-

(μmol/L) 

Cain 

Spring 12/20/2004 10 7.9 2.4 - 15 700 

A 10 8.7 1.2 - 53 40 

B 10 8.9 3.1 - 23 40 

C 28 8.7 3.6 - - - 12/21/04 

10 7.1 1.0 - 2.8 70 

D 2/16/2005 10 8.5 0.9 9.0 - - 

5/27/2005 24 8.8 1.0 9.5 - - 

I 10 8.5 0.7 7.9 0.20 77 

J 

2/16/2005 

10 8.2 0.5 4.8 0.13 81 

L 10 8.7 3.6 - <0.002 26 

M 

12/21/04 

10 7.6 5.9 29 34 <17 

28 8.3 6.1 - - - 

MB 

5/27/2005 

22 8.3 1.2 - - - 

12/21/04 12 8.4 1.3 - 45 52 

N 

5/27/2005 - - - 12 - - 

2/16/2005 10 8.6 2.9 11 - - O 

5/27/2005 25 8.7 3.8 13 - - 

P 5/28/2005 16 7.2 0.7 - - - 

Q 2/23/05 10 7.7 1.2 - - - 

R - - - 7.6 - - 

S 

5/26/05 - - - 2.8 - - 

06RP1 15.3 8.0 5.10 9.70 41 280 

06RP3 

12/14/06 

15.0 8.0 1.53 10.9 3.2 170 

06RP5 15.3 8.0 1.20 11.0 0.82 420 

06RP6 16.4 8.4 19.1 26.3 170 1,800 

06RP7 15.1 8.4 9.37 18.2 78 690 

06RP8 

12/15/06 

12.3 8.1 8.11 17.9 62 760 

31 
Table 7. Concentrations and properties of dissolved organic matter in saline 

ground water seepage sampled in Bear Valley, December 2006. 

Sample Location UV Absorbance1 DOC (mg/L C) SUVA2542 FI3 

06RP01 0.049 2.4 2.1 1.45 

06RP3 1.484 29.9 5.0 1.24 

06RP5 0.939 22.2 4.2 1.29 

06RP6 1.986 50 4.0 1.37 

06RP7 0.225 7.6 3.0 1.39 

06RP8 0.594 14.2 4.2 1.35 

1

Ultraviolet Absorbance measured at a wavelength of 254 nm. 

2

SUVA254 = UV(254nm) / DOC (mg/l) * 100. 3

FI = Fluorescence Index (ratio of the emission at 470nm / emission at 520nm, 

when excited at 370nm.) 

The presence of higher HgT concentrations in unfiltered drainage from the mined area compared 

to unfiltered saline spring water may discriminate mine versus natural sources of Hg. For 

example, in May 2005 at locations R (close to the Petray north pit) and D approximately 1,050 

in-stream meters downstream along the north fork of tributary 1, HgT in water was 110 ng/L and 

75 ng/L, respectively (table 4). These concentrations are higher than HgT measured in six of 

seven spring waters collected in December 2006. Surface water collected 1,250 meters further 

downstream from location D (2,300 stream meters from the Petray north pit) along tributary 1 

had an even higher HgT concentration (590 ng/L, location N) in May 2005. At these same 

locations and times, however, the proportion of 0.45 μm-filtered Hg (HgF) in drainage from the 

mined area was approximately a third that in saline ground water. These results suggest that 

identification of mine-related drainage dominance over saline ground water can be discriminated 

on the basis of higher HgT concentrations (100s ng/L) with smaller fractions of HgF. 

The Hg species in drainages from the mined area include cinnabar, metacinnabar, and Hg(II) that 

is loosely bound to particles and subject to carbonate-mediated release and sequestration. 

Filtered Hg (HgD) appears to be correlated to HgT (fig. 9), which suggests that physicochemical 

regulation of carbonate mineral cements by stream flow strongly affects both the release of 

colloids and dissolution of Hg(II) sorbents in Bear Creek tributaries (see section 1.4 and fig. 3). 

32 
0

0.2

0.4

0.6

0.8

1

1.2

400 450 500 550 600

Intensity

Emission wavelength (nm)

06RP-8

06RP-6

06RP-5

06RP-7

06RP-3

06RP-1

b

Figure 7. (a) Excitation-emission matrix for Bear Valley ground water seepage location 06RP1. (b) 

Fluorescence emission spectra at 370 nm excitation wavelength, as indicated by the dashed line in (a). 

[The vertical guide lines indicate the emission wavelengths used 

to compute the fluorescence index (FI = I470 / I520)] 

33 
1.2

1.3

1.4

1.5

23456

FI

SUVA254

FI = 1.6 - 0.07 SUVA254

R2 = 0.92

F ratio = 23.4, Prob > F = 0.008

Figure 8. Apparent correlation between DOC the fluorescence index and 

SUVA254. 

[The ANOVA statistics shown in the figure suggest that it is highly 

unlikely (only 0.8 percent of the time) that a correlation this large (R2

 = 

0.92) could happen by chance alone] 

-0.5

0

0.5

1

1.5

2

2.5

3

0 0.5 1 1.5 2 2.5 3

log HgF

log HgT

R2 = 0.65

F ratio = 20.4

Prob >F = 0.0009

Figure 9. Correlation between log10 transforms of filtered (<0.45 μm) and 

total Hg in surface water in streams draining the Petray Mine and Bear 

Valley. 

[The ANOVA statistics shown in the figure suggest that it is highly 

unlikely (only 0.09 percent of the time) that a correlation this large (R2

 = 

0.65) could happen by chance alone] 

34 
Although the Petray Mine has the potential to export Hg to Bear Creek, on a time-integrated 

basis, saline ground water is an appreciable and possibly dominant source of Hg. The 

concentrations of HgT in filtered spring water in Bear Valley were often similar to unfiltered 

water and suggest that the majority of Hg is dissolved or colloidal. Dissolved organic matter 

(DOM) may help solvate Hg (for example, by inhibiting precipitation of Hg or carbonatecementation of Hg-bearing particles). Both dissolved and colloidal Hg are mobile in subsurface 

flow and seepage runoff. The springs in Bear Valley are closer to Bear Creek than are the 

Rathburn and Petray Mines. A similar hypothesis was proposed by Domagalski and others 

(2004b) for waters in Sulphur Creek (another tributary to Cache Creek). They also hypothesized 

that saline geothermal springs elevated Hg concentrations in Sulphur Creek. Additional 

monitoring, and particularly hydrogeological investigations (see section 4), is required to 

determine whether saline ground water is an appreciable or dominant source of Hg to Bear 

Creek. 

Pore water sampling provided the most reliable method to-date to characterize connate water 

with respect to Hg content and methylation potential. In December 2006, pore water sampling 

devices were deployed at locations where conditions appeared conducive to Hg accumulation 

and methylation. Sampling of pore water reduces dispersion and oxidation, which can change the 

speciation of major constituents such as sulfur and iron that, in turn, can affect the speciation of 

Hg. In addition, since pore water is not exposed to sunlight, sampling of pore water minimizes 

photochemical degradation of MMHg. Rainfall consisted of intermittent drizzling at the time of 

sampling in December, 2006. Although the wet season had just begun and surface runoff in the 

tributaries from the mine area was not evident (a relatively drier water year), most of the nearsurface pore water and surface water sampled on December 13-14, 2006 consisted of less than 50 

percent connate waters and the pore waters were predominantly meteoric (table 4). 

Despite having less dissolved Hg in pore fluids compared to surface water, the shallow 

subsurface may more efficiently produce MMHg. In most cases, the concentration of dissolved 

Hg in pore waters near active Bear Valley seeps (9 to 19 ng/L, mean 14 ng/L; n = 3) was about 

half that measured in surface water (0.04 to 110 ng/L, mean 31 ng/L; n = 7; table 4 and fig. 2). 

However, fractions of dissolved MMHg relative to HgF were one to two orders of magnitude 

larger in pore water compared to surface water. The simplest explanation for the difference in 

HgD between surface and pore water is that sulfide [S(-II)] generated in the subsurface by sulfatereducing microorganisms (2 to 4 μM S(-II), table 4) reacted with Hg(II) to form metacinnabar 

(β-HgS(s)). Another possible, but less likely reason for the observed elevated HgD concentrations 

in surface versus pore water is that 0.45 μm filtrates of surface water contained more Hg-bearing 

colloids than the 0.1 μm pore water filtrates. Biogeochemical conditions in the near-subsurface 

were more conducive to Hg(II) methylation than in surface water environments. Sulfate-reducing 

microorganisms were not likely living in oxygenated surface water but were evidently present in 

the subsurface, on the basis of sulfide pore water measurements. Therefore, at least one known 

pathway for MMHg formation likely differs appreciably between the surface and subsurface 

environment. Although sulfide formed in the subsurface may sequester a fraction of Hg(II) as βHgS(s), dissolution of β-HgS(s) (Ravichandran and others, 1998; Waples and others, 2005) or 

partial inhibition of Hg(II) precipitation (Ravichandran and others, 1999) by DOM may provide 

a persistent source of dissolved Hg(II) to methylating microorganisms. The high concentrations 

of MMHg in pore waters at the north and south end of the Bear Fault indicate that pore fluids in 

the near surface ground water east of the fault are a potentially significant source of MMHg to 

35 
Bear Creek. Effluent from the springs maintains a large wetland area east of the fault (in the area 

around locations BF8 in fig. 2c) and subsurface flow likely contributes MMHg to Bear Creek. 

4. Conclusions 

4.1 General conclusions 

The Rathburn and Petray Mines and saline spring waters are sources of Hg to Bear Creek. An 

understanding of the relative importance of these two sources of Hg and Hg transport requires an 

understanding of the hydrology in the Bear Valley drainage basin. Although the Petray Mine 

and, to a lesser extent the Rathburn Mine, has the potential to yield mobile forms of Hg to Bear 

Creek tributaries, saline ground water is an appreciable and possibly dominant source of Hg. 

While we infer from interpretation of geochemical characteristics of water and sediment that 

carbonate plays a major role in the physicochemical transport of Hg, the data are not adequately 

sensitive to discern the relative contributions of meteorically derived drainage from the mined 

area versus saline ground water sources of Hg to Bear Creek. The results of this study provide a 

basis for developing a conceptual model to effectively reinvestigate the Bear Valley region in 

order to predict the effects of cleanup of the Rathburn and Petray Mines on Bear Creek. Further 

study of the Bear Valley drainage basin in light of the conclusions provided below is required to 

achieve such predictive capability. 

The concentration of Hg in waste materials at the Rathburn Mine varies and depends on material 

type. While calcines contain much higher Hg concentrations than waste rock and serpentinite, 

they comprise a minor fraction of the exposed material at the Rathburn Mine and are absent at 

the Petray Mine. The majority of easily eroded Hg-contaminated materials at both mines consist 

of waste rock and Hg-enriched altered serpentinite and quartz-chalcedony veins. 

Although all three pits at the Petray Mine have potential to release Hg-enriched sediment into the 

adjacent drainages, not all waste piles in the vicinity of each pit appear to have transport 

pathways emanating from them (section 3.1). Two of three open pits at the Petray Mine release 

Hg-enriched sediment and account for the high Hg concentrations in sediments in the north fork 

of tributary 1. Erosion of waste rock from a large dump northeast of the south pit contributes Hgenriched sediment to tributary 3. 

Although high concentrations of Hg in sediment persist in the north fork of tributary 1 

downstream from the central and north Petray Mine pits, Hg concentrations vary considerably 

and do not consistently decay from the source. Cinnabar and metacinnabar are likely the primary 

Hg phases in sediments of the north fork of tributary 1. Evidence suggests that sediment in 

tributary 1 is diluted by sediment (primarily calcium carbonate) transported from the south fork 

of tributary 1. Spring water entering the south fork of tributary 1 downstream of the Petray South 

pit is cementing particles in the stream channel: precipitated carbonate retards the transport of 

Hg-enriched sediment. 

While pH, specific conductivity (SC), and alkalinity certainly reflect the influence of saline 

spring water in stream channels draining the Petray Mine and within Bear Valley, they do not 

clearly indicate the proportion of meteoric versus high-salinity ground water. Ambiguity arises 

from the dissolution and precipitation of carbonate-bearing solids, which affects all of these 

parameters. However, elements such as Cl and Na are not affected by carbonate precipitation and 

36 
redox reactions and, therefore, are useful in establishing, under contrasting hydrological 

conditions, water origin and dominance. The most saline spring water is present in the central 

part of the Bear Fault and is likely representative of a nearly pure connate ground water endmember. This water also has the highest concentration of other transition elements such as B, 

Ca, K, Mg, Rb, Sr, and W. Other springs along the Bear Fault and at the travertine terrace consist 

of variable proportions of saline ground water and meteoric water. Surface waters are 

predominantly meteoric except where the effluent from saline springs mixes with surface water 

such as in the south fork of tributary 1 and where this tributary intersects the Bear Fault. This 

investigation would be greatly assisted by contemporaneous hydrological and geophysical study 

of the system to understand the saline ground water reservoir and its response to seasonal 

changes in meteoric water flow. 

 

Spring waters that consist of variable proportions of near-surface meteoric water and saline 

ground water have unique DOC characteristics. The pure saline ground water end member has 

the highest DOC. Limited data indicate that DOC is correlated with the proportion of saline 

ground water present in spring waters along the Bear Fault. This investigation characterized 

DOC using indirect measures of aromaticity, SUVA254, excitation emission matrices, and 

fluorescence indices (EEM/FI), but was limited by the possibility that springs and pore fluids 

also contained a component of terrestrially derived organic carbon present in near-surface 

meteoric ground water (that is, from bovine manure and degradation of plants). At the time of 

spring water sampling, no surface flow in the tributaries and low subsurface flow was likely 

because only minor rainfall had occurred in the drainage basin after the prolonged dry season. 

Pore waters sampled along the north and south end of the Bear Fault are meteoric and have 

relatively high DOC and a larger fraction of aromatic carbon likely derived from a terrestrial 

source. SUVA254 and EEM/FI are likely sensitive to end-member water mixing as well as 

contributions from terrestrial derived sources but additional data is necessary to characterize the 

ground water end member and other DOC sources in meteoric water. 

The presence of higher HgT concentrations in unfiltered drainage from the mined area compared 

to unfiltered saline spring water may discriminate mine versus natural sources of Hg. The Hg in 

drainage from the mine area is primarily particulate, including cinnabar, metacinnabar, and 

Hg(II) that is loosely bound to particles and subject to carbonate-mediated release and 

sequestration. While HgF is always less than HgT because of transport of cinnabar, metacinnabar, 

and colloidal Hg(II)-mineral sorption complexes, the correlation of HgF to HgT (fig. 8) suggests 

that hydrological processes that release colloids also partially dissolve sorbents and carbonate 

cement in the stream channels. 

The majority of Hg in Bear Valley ground water is dissolved and/or colloidal. Dissolved organic 

matter (DOM) may help solvate Hg (for example, by inhibiting precipitation of Hg or carbonatecementation of Hg-bearing particles). Despite having less dissolved Hg compared to surface 

water, the shallow subsurface may more efficiently produce MMHg. Biogeochemical conditions 

in the near subsurface were more conducive to Hg(II) methylation than in surface water 

environments. Sulfate-reducing microorganisms present in the subsurface resulted in the net 

production of MMHg. Although sulfide formed in the subsurface may sequester a fraction of 

Hg(II) as HgS(s), dissolution of HgS(s) by DOM may provide a persistent source of dissolved 

Hg(II) to methylating microorganisms. Elevated MMHg concentrations in shallow subsurface 

pore water at the north- and south-end wetlands of the Bear Fault suggest that these areas could 

be a source of MMHg to Bear Creek. 

37 
Subsurface flow, seepage, and runoff of ground water throughout Bear Valley are expected to 

strongly influence the aqueous carbonate and organic chemistry of Bear Creek. Aside from Hg 

transported through Bear Creek, carbonate and DOC likely transport more conservatively than 

Hg and so may affect biogeochemical transformations of Hg in larger Cache Creek tributaries 

(for example, Sulphur Creek). The importance of the Bear Valley system to the Cache Creek 

basin may be based on DOC and carbonate rather than just the Hg transported from it. 

4.2 Predicting the efficacy of a removal of Rathburn and Petray Mine waste 

The implication of the EE/CA currently being undertaken by the BLM is that Hg-contaminated 

mine waste would be removed or immobilized on-site at the Rathburn and Petray Mines. If such 

actions would eliminate any future release of Hg from the mine sites, it is necessary to predict 

the transport of Hg from contaminated sediment in Bear Creek tributaries, primarily tributaries 1 

and 3 (due to past mine release) and from ground water discharge along the Bear Fault. This 

highly complex system is not likely to be amenable to Hg transport modeling, given limited 

resources and the technical capabilities required. Therefore, to what qualitative or semiquantitative extent can Hg transport be predicted? 

Predictive capability depends on achieving an understanding of Hg concentrations and phase 

distribution and perhaps further understanding of the chemical speciation in the Bear Valley 

system. Two distinct, end-member sources; namely, meteoric water and saline connate ground 

water, contribute to the chemistry of springs and surface waters. The springs have a larger 

component of saline ground water: a carbonate spring in the central part of the Bear Fault has the 

highest component of this source. All the surface waters are predominantly meteoric with a 

significant component of saline ground water being present only in Tributary 1 because of the 

presence of effluent from saline springs. The proportions of each end member contributing to the 

flow in Bear Creek can be estimated under a suitable range of climatic and hydrological 

conditions using geochemical parameters that are conservative. However Hg is non-conservative 

and dependent on several chemical and physical processes. 

Because of the multiple sources of mercury and the complexity of the drainage basin between the 

Rathburn and Petray Mine areas and Bear Creek, establishing a post-remediation sampling 

protocol to document the effects of the remediation of the mines will be challenging. This study 

provides data and considerations that establish a scientific framework to develop a conceptual 

model that supports a rigorous future investigation of Hg transport post remediation. Further 

studies are also needed to determine processes that control Hg release and methylation in the 

Bear Creek drainage basin. 

Acknowledgements. This report was improved through careful and forthright reviews by Dr. 

Roger Ashley (Scientist Emeritus, USGS) and Laurie Balistrieri (USGS/University of 

Washington). We also acknowledge assistance with formatting and style from John Galloway 

and Jane Ciener that clarified the report’s presentation. 

6. References 

Aiken, G.R., 1992, Chloride interference in the analysis of dissolved organic carbon by the wet 

oxidation method: Environmental Science & Technology, v. 26, p. 2435-2439. 

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41 
Appendix 1. Photographs 

Figure 10. Petray Mine, May 2005 (denoted as photo 10 in fig. 2). 

Figure 11. Near location M, looking eastward toward Bear Valley, May 2005 (denoted as photo 11 

in fig. 2). 

42 
Figure 12. Location M, May 2005 (denoted as photo 12 in fig. 2). 

Figure 13. South fork of tributary 1, May 2005 (denoted as photo 13 in fig. 2). 

43 
Figure 14. Algal biomass in Dead Shot Creek, May 2005 (denoted as photo 

14 in fig. 2). 

Figure 15. Cinnabar ore in alterred serpentine at 

the Petray Mine, May 2005. 

44 
Figure 16. Alterred serpentine at the Rathburn Mine (a), ppen pit at the Petray Mine (b), Petray 

fault zone (c), north Pit at the Petray Mine (d), waste rock at the Rathburn Mine (e), alterred 

serpentine at the Petay Mine (f), silica-carbonate veins at the Petray Mine (g), and terraced 

topography at the Petray Mine (h). (taken May 2005). 

45 
Figure 17. Travertine along tributary 1, December 2004. 

Figure 18. Travertine “falls” along tributary 1, December 2004. 

46 
Figure 19. Ground-level view of vegetated land along fault 

zone depicted in figure 2, December 2006. 

Figure 20. Large seep of connate ground water, including 

evolving gas (presumably CO2, based on its lack of any 

detectable odor), December 2006. 

47 
Figure 21. Unsaturated carbonate-rich mud and grass coated 

with efflorescent salt, December 2006. 

Figure 22. Carbonate and iron-rich ground water seepage. Note also faint 

regions of green material and the nonuniformity of iron (orange) and green 

coloration, December 2006. 

48 
Figure 23. Tributary 1 on December 13, 2006 at dusk (photo levels 

enhanced). Note white mineral coating along stream channel. 

Figure 24. Ground water seepage and resulting drainage channel. Shiny 

object is an aluminum foil-covered pore water sampling device, December 

2006.

49