D ata orr lbe chemical 

quality of base flow 

, 'from 33 small streams 

that drain basins of 

differitlg land-use type and intensity 

within the Croton watershed were 

collected seasonally for 1 year to 

identify and characterize the quality 

of ground-water contributions to 

surface water. The watershed 

includes twelve of New York City's 

water-supply reservoirs. Baseflow 

samples were collected a minimum 

of three days after the most recent 

precipitation and were analyzed for 

major ions, boron, and nutrients. 

Findings-

• Concentrations of selected 

chemical constituents in baseflow 

were strongly affected by the 

predominant land use in a given 

basin. Land uses included forested 

undeveloped, unsewered residential, 

sewered residential, and agricultural 

(horse and dairy farms). 

• A positive linear relation was 

indicated for chloride concentration 

in baseflow and the basin's annual 

U.S. Geological Survey 

Department of the Interior 

rate of road-salt application (or 

density of two-lane roads). Chloride 

concentration exhibits a relatively 

stable relation to road-salt 

application rate or two-lane road 

density throughout the year. 

• Positive linear relations were 

indicated for nitrate concentration 

in baseflow and the basin's 

unsewered-housing density. Nitrate 

is characterized by a different 

relation to unsewered-housing 

density for each season, with the 

highest observed nitrate 

concentrations during the winter 

and the lowest concentrations 

during the summer. 

• Baseflow nitrate concentrations 

in sewered basins, and in unsewered 

basins with riparian wetland buffers 

between residential development and 

the stream, were lower than 

concentrations predicted from 

unsewered-housing density. 

Croton watershed stream under baseflow conditions 

WRIR 99-4173 

March '2000 
22' 

30" 

15' 

45' 37'30" 

EXPLANATION 

DRAINAGE AREA OF STREAM SITE- number is local basin number 

referred to in table 1 and figure 3. 

• STREAM BASEFLOW SAMPLING S 

- • • WATERSHED BOUNDARY 

0 

0 

Base from U.S. Geological Survey 

digital data, 1:100,000, 1983 

30' 

1-

::::> 

() 

;:: 

(.) 

w 

z 

z 

0 

() 

\ 

' \ 

Figure 1. Locations of the 33 baseflow-sampling sites and their respective basins within 

the Croton watershed in southeastern New York. 

2 

INTRODUCTION 

The Croton watershed, a 374 

mF basin in southeastern New 

York, contains 12 reservoirs 

that together provide about 10 

percent of New York City's 

water supply (fig. 1). The area 

is underlain by igneous and 

metamorphic bedrock mantled 

by a discontinuous cover of 

till of variable thickness in 

upland areas, and alluvium, 

peat-muck, and outwash in the 

main valleys. The watershed, 

which is directly north of the 

City and encompasses parts 

of Westchester, Putnam, and 

Dutchess Counties, has 

undergone significant suburban 

development. Undeveloped 

forested land is still 

predominant (57 percent) in 

the watershed, followed by 

residential development (25 

percent) and agriculture 

(including fallow fields and 

parks) (7 percent) (Linsey and 

others, 1999). The population 

of the watershed in 1990 was 

about 176,000 (U.S. 

Department of Commerce, 

1990). Deterioration of 

reservoir water quality and the 

possibility of federally 

mandated filtration of the water 

supply has resulted in increased 

regulation, monitoring, and 

investigation of watershed 

processes and human activities. 

In 1996, the United States 

Geological Survey (USGS) 

began a cooperative study with 

the New York City Department 

of Environmental Protection 

(NYCDEP) to assess groundwater quality within the 

Croton watershed. 

Ground-water discharge is a 

major contributor to streams 

that feed New York City's 

water-supply reservoirs. The 
relatively stable flow of streams 

during periods of little or no 

precipitation, referred to as baseflow, 

is sustained totally or in large part Table 1. Physical and land-use characteristics of stream basins 

by ground-water discharge (except in sampled in Croton watershed in southeastern New York, 1996-97. 

streams downgradient of reservoirs [mi2, square miles; rnilmi2, miles per square mile; tons/mi2, tons per square and lakes) (fig. 2). Ground-water mile; (tons/mi2)/yr, tons per square mile per year. Basin and station 

discharge accounts for at least 60 locations are shown in fig. 1.] 

percent of total annual streamflow, Basin characteristics 

as indicated by physical separation Local USGS Road Road-salt- Housing 

of stream-discharge hydro graphs basin surface-water Are/ densi~ applicatio~ rate dens it~ 

no. station no. jrl2fJ* .. (mi/mi_) [(to~s/mi)lYJl _ (per mi ) 

(Rutledge, 1993) from two well- ·. ,..,"{Tt1'. ~-

0137448710 1.55 2.61 96.6 49.8 

drained basins (minimal wetlands 2 0137448720 0.32 1.19 43.9 12.5 

and surface-water impoundments) 3* 0137448605 1.33 4.04 149.3 207.8 

within the Croton watershed. Thus, 6 0137448812 0.52 1.38 51.0 25 the chemical quality of baseflow 

7 0137491450 0.06 14.20 474.8 600 from a given stream basin gives an 

indication of the composite quality of SA 0137463580 0.74 1.26 43.2 35.1 

the shallow ground water within that 9A 0137449120 0.28 0.61 22.4 17.9 

basin. Baseflow quality can differ 10 0137462530 0.34 7.69 691.5 82.4 

from ground-water quality; nutrient 12 0137454950 1.27 2.43 155.8 2.4 

concentrations in ground water may 13 0137454960 1.16 0.92 88.0 0 

be diminished by bacterially 18 0137449520 0.11 14.96 553.7 839.3 

mediated chemical transformations 19 0137449510 0.26 0.59 21.9 38.8 

in and immediately below the 20 0137449440 0.15 25.76 980.9 1,020.1 

streambed (hyporheic zone) as the 21 0137449435 0.52 7.04 311.3 427.5 

water discharges to the stream, and 22 0137449445 0.57 15.18 576.9 714.3 

the concentration of dissolved 23 0137449487 0.58 1.88 69.5 67.1 

nutrients within the stream can be 

27A 01374853 0.17 7.91 363.5 494.1 further diminished through algal and 

plant uptake. Nevertheless, baseflow 28A 01374849 0.26 12.17 493.3 621.2 

samples provide a more accurate 29 * 0137494905 0.23 1.65 64.2 121.7 

and cost -effective measure of the 31* 0137494710 0.25 14.07 520.5 944.9 

chemical quality of ground-water 33A 0137497652 0.23 5.60 164.1 182.6 

contributions to the reservoirs than 35 01374790 0.22 1.32 48.8 4.5 

would ground-water samples from a 36A 01374783 0.15 0.00 0 0 

regional well network. 36B 01374785 0.58 1.03 38.2 13.8 

This report documents the effects 37 0137470990 0.43 8.94 330.9 556.8 

of land use on baseflow water quality 38 01374889 1.08 1.12 41.5 2.8 

at the 33 small (first- and second- 39 01374893 0.67 0.27 10.2 1.5 

order) streams that drain basins 41 0137490520 0.12 9.15 335.8 208.3 

representative of the different type( s) 42A 0137498830 0.49 3.03 98.9 61.2 

and intensities of land use within the 

44At 0137498290 0.19 12.14 482.2 363.2 Croton watershed (fig. 2, table 1). 

Emphasis was given to identification 50 A 0137464640 0.06 25.14 820.2 1,100 

of the effects of unsewered residential 52 0137449430 0.19 5.25 355.8 347.4 

development on the chemical quality 54 A 01374936 0.25 6.93 256.3 308.9 

of baseflow because of the prevalence * Sewered 

of this land use in the watershed. t 30-percent sewered 

Small basins with drainage areas of tt Horse and dairy farms 

0.06 to 1.55 mi2 were chosen over 

larger stream basins for: 1) better 

3 
control over the type and intensity 

of land use( s) within each basin, 

and 2) rapid return to baseflow 

conditions after storms (within 3 

days; based on flow records from 

USGS gaging station 0137497600, 

which has a drainage area of 3.01 

mF). Other criteria used in the 

selection of study sites included 1) the 

absence of upgradient point-source 

discharges (typically from sewagetreatment plants), (2) the absence 

of up gradient surface-water bodies 

(other than small ponds or wetlands) 

that could store and contribute water 

NOT TO SCALE 

derived from previous storms or 

increase biological activity (nutrient 

uptake) by increasing surface-water 

residence times, and 3) basin-wide 

distribution of sites, including 

representation of major metamorphic 

and igneous bedrock types. 

The stream network was sampled 

four times (July-August 1996, 

October-November 1996, January 

1997, and May 1997) to document 

seasonal baseflow water quality at 

each site. Grab samples collected 

from each stream at least 3 days 

after the most recent storm were 

EXPLANATION 

analyzed for major ions, boron, and 

nutrients (nitrogen and phosphorus 

compounds) by the USGS National 

Water Quality Laboratory in Arvada, 

Colo. Field measurements included 

specific conductance, pH, dissolved 

oxygen, and temperature. The field 

data and results of the laboratory 

analyses are given in Butch and 

others (1998). Analyses of nitrate 

plus nitrite are herein referred to as 

nitrate because nitrite is an unstable, 

low concentration (or undetected), 

intermediate product of nitrification 

that is ultimately converted to nitrate. 

POTENTIAL CONTAMINANT SOURCES UNCONSOLIDATED SEDIMENT 

0 UNSATURATED 

4 

<D Septic systems SATURATED 

® Road salt ~ BEDROCK (FRACTURED) 

@ Lawn fertilizers .-:JI"" DIRECTION OF GROUND-WATER FLOW 

@) Animal waste ~ RECHARGE 

® Pesticides ____JZ__ WATER TABLE 

Figure 2. Hypothetical stream basin within the Croton watershed showing ground-water flow toward the stream 

along with potential contaminant sources associated with residential development and agriculture. 
DOES LAND-USE TYPE AFFECT BASE FLOW CHEMICAL QUALITY? 

Baseflow chemical quality differs among streams that drain basins dominated by one of four major land uses within 

the Croton watershed: 1) undeveloped forested, 2) unsewered residential, 3) sewered residential, and 4) agricultural 

(herein referred to as horse and dairy farms). Chemical quality of baseflow from the four basins with the highest 

percentage or intensity of each of these land uses is depicted in figure 3 and discussed below. 

Undeveloped Forested (Basin 36A) 

Basin 36A (fig. 1) is the only 

entirely forested basin sampled 

during this study; therefore, its 

baseflow chemical quality reflects 

natural undisturbed conditions and 

provides a standard for comparison 

Unsewered Residential (Basin 20) 

Each home in unsewered 

residential areas is served by an 

on-site septic system that ideally 

discharges wastewater to the 

unsaturated zone, where it percolates 

downward to the water table and 

becomes shallow ground water. The 

basin with the greatest density of 

septic systems is basin 20 (fig. 1), 

which contains part of a lakeside 

community and is characterized by 

high densities of houses (1 ,020 

houses/mF) and roads (25.8 mi /mi2

). 

Specific conductance and the 

concentrations of all constituents 

except the phosphorus species 

(which were below detection limit) 

exceeded those found in the 

baseflow of the undeveloped forested 

basin (fig. 3). The specific 

conductance of baseflow from basin 

20 exceeds that in the undeveloped 

forested basin baseflow by a factor 

of 14. Baseflow from basin 20 also 

has higher average concentrations 

of chloride (93x), sodium ( 47x), 

sulfate (2x), boron (5x), nitrate (25x) 

and ammonia (greater than 4 x) than 

that in the forested undeveloped 

basin. These values reflect the high 

intensity of unsewered residential 

with other basins. The baseflow in 

this basin has the lowest average 

concentrations of all constituents 

studied--the median concentrations 

of ammonia, total phosphorus, and 

orthophosphate were below detection 

limits (fig. 3). Increases in specific 

conductance and chemical 

development in basin 20. 

High concentrations of chloride 

and sodium are primarily attributed to winter application of salt 

on the many roads that accompany intensive residential development, as discussed later. Less 

elevated levels of chloride and 

sodium are also characteristic of 

domestic wastewater. 

Sulfate, boron, ammonia, and 

organic nitrogen are initially present 

in septic effluent (Robertson and 

others, 1991; Harmon and others, 

1996;LeBlanc, 1984), butin 

properly working septic systems, 

most organic nitrogen and ammonia 

are oxidized to nitrate during 

transport through the unsaturated 

zone to the water table. Therefore, 

nitrate is the dominant nitrogen 

species in ground-water discharge to 

streams, although reduced nitrogen 

species can persist in ground water 

and surface water, as in basin 20 (fig. 

3; table 1), where the unsaturated 

zone is thin or of low permeability, 

or where septic systems fail. 

Fertilizers applied to lawns are a 

secondary source of nutrients that 

may vary with lawn size and 

constituents in the other basins 

relative to those in the forested 

undeveloped basin (36A), are 

expressed as an "enrichment factor" 

representing the number of times 

by which the value for the given 

developed basin exceeds that for the 

forested undeveloped basin. 

neighborhood affluence. The high 

housing density, small lot size, and 

hillside setting within basin 20 limits 

lawn area and thus may limit 

fertilizer application. Nitrate 

concentrations can be diminished 

through 1) denitrification as ground 

water passes through organic-rich 

sediments or wetland areas prior to 

reaching a stream and 2) biologic 

(algal and plant) uptake after 

discharge to the stream. 

Phosphorus species typically found 

in septic effluent were not detected 

in baseflow samples from basin 20; 

their absence is attributed to mineral 

precipitation and strong adsorption 

onto sediments upon discharge from 

septic systems to ground water. 

These processes greatly retard 

downgradient migration relative to 

the velocity of the local ground 

water. Phosphorus species may reach 

streams where sorption sites are 

saturated; this is most likely to occur 

where septic systems have been in 

service for many years (Harmon 

and others, 1996). Phosphorus 

concentrations in ground water 

discharging to streams can be 

decreased through uptake by algae 

and plants. 

5 
Sewered Residential Areas (Basin 31) 

Sewered residential areas are 

served by sanitary sewers that route 

domestic wastewater away from these 

areas to a sewage-treatment plant 

that typically treats and discharges 

the effluent to a point downstream 

of the basin. In instances where 

sewering post -dates the housing, a 

small number of houses may still be 

served by septic systems. Basin 31 

has the highest density of housing of 

the three sewered residential basins 

sampled, but the housing density 

(945 houses/mi2

) and road density 

(14.1rnilmi2

) are lower than in basin 

20 (unsewered) because the house lots 

are larger. 

Constituent concentrations were 

higher in baseflow from the sewered 

Horse and Dairy Farms (Basin 1) 

Most horse and dairy farms are 

in the eastern part of the Croton 

watershed. Basin 1 contains the 

highest percentage of this land use 

(about 12 percent of the basin) 

sampled in this study; the remaining 

land uses in this basin are fallow 

fields, undeveloped forested, and lowdensity unsewered residential. The 

housing density (50 houses/mi2

) and 

road density (2.6 mi/mi2

) are much 

lower than in the residential basins. 

The average water quality of 

baseflow from basin 1 is consistent 

with the types and intensities of 

land use previously described. The 

low enrichment values for specific 

conductance (4x), chloride (6x), and 

sodium (3x) reflect the low number 

of roads in an agricultural area and 

the low enrichment values for sulfate 

(1.2x) and boron (1.3x) reflect the 

low housing and septic-system 

density in the area. All nutrient 

concentrations in baseflow from basin 

1 are among the highest of any of the 

basins sampled and appear to indicate 

animal-waste inputs. The enrichment 

values for ammonia (>5x), total 

6 

residential basin (basin 31) than in 

the undeveloped forested basin, but 

were lower (except for the phosphorus 

species) than in the unsewered 

residential basin. This basin's 

enrichment values for specific 

conductance (9x), chloride (37x), and 

sodium (18x) are much less than those 

from the unsewered residential basin, 

largely because the sewered basin's 

road density is lower; the general lack 

of septic system discharges could also 

contribute to lower concentrations. 

The enrichment factors for sulfate 

(1.7x) and boron (4x) are similar to 

those from the unsewered residential 

basin, whereas the enrichment factors 

for nitrate (7x) and ammonia (>2x) 

are lower, and the orthophosphate 

value (>3x) is higher. Potential 

sources of these constituents include 

ammonia and organic N (> 1.5x), and 

total phosphorus (>4x) are greater 

than those for the two residential 

basins (20 and 31). The nitrate 

enrichment value (9x) is similar to 

that for the sewered residential basin 

and is well below the value for 

the unsewered residential basin. 

Orthophosphate enrichment (>2x) in 

this basin is similar in magnitude to 

the value for the sewered residential 

basin (basin 31). 

Elevated concentrations of 

nutrients in baseflow from basins 

with horse and dairy farms may 

be derived from sources other than 

shallow ground-water discharge; for 

example, animal waste can enter 

streams directly where livestock are 

allowed to roam in and around the 

streams, and small, contaminated 

ponds or temporary impoundments 

can gradually drain into streams. 

Nutrients from these surface sources 

can enter streams in dissolved and 

particulate forms whereas nutrients in 

ground-water discharge presumably 

are only in dissolved form. 

Dissolved phosphorus in a stream 

can be removed through sorption 

onto sediment particles or through 

leaky sanitary sewer lines, septic 

discharges from the few homes not 

connected to sanitary sewers, and 

lawn fertilizers. Leakage from sanitary 

sewer lines is a likely source because 

the main sewer line in the basin 

closely follows the stream course, 

in which case 'breakthrough' of 

orthophosphate from sewer line to 

stream might occur. The elevated 

boron concentration is probably 

derived from the first two sources 

because (1) it is typically not an 

ingredient of homeowner-applied 

fertilizers (although it may be present 

in professionally applied fertilizers), 

and 2) bedrock is not an important 

source, as indicated by low 

concentrations of boron in baseflow 

samples from undeveloped forested 

basins with different bedrock types. 

uptake by algae, thereby becoming 

particulate phosphorus (measured as 

part of total phosphorus) that is 

largely unavailable to the stream until 

scoured and suspended by higher 

flows. Dissolved phosphorus can also 

be removed from the stream through 

uptake by aquatic plants. 

Elevated concentrations of 

reduced nitrogen species (organic 

nitrogen and ammonia) in baseflow 

are indicative of a surface- or 

ground-water source that is near 

the stream; this implies short 

residence time for these nitrogen 

species at land surface or within 

a shallow ground-water environment 

prior to reaching the stream. 

Otherwise, ammonia is transformed 

(through nitrification) to nitrate, a 

stable form of nitrogen, under 

oxidizing conditions. Particulate 

organic nitrogen, like particulate 

phosphorus, may be derived directly 

from surface sources or from 

discharge of dissolved forms from 

ground water with subsequent 

uptake by algae. Dissolved nitrogen 

species may be derived from either 

ground-water discharge or surface 

sources (impoundments). 
0 200 400 600 800 1 000 1200 

SPECIFIC CONDUCTANCE, IN MICROSIEMENS PER CENTIMETER 

Chloride 

Sodium 

Sulfate 

Boron (ug/L) 

0 20 40 60 80 100 120 140 220 240 

Nitrate plus * 

Nitrite as N 

Ammonia and * * Organic N, Total * * 

0 0.5 1.5 2 2.5 

Ammonia as N * 

Orthophosphorus * 

asP * 

Phosphorus, * * Total * * 

0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 

CONCENTRATION, IN MILLIGRAMS PER LITER 

EXPLANATION 

• UNDEVELOPED FORESTED (BASIN 36A) • SEWERED RESIDENTIAL (BASIN 31) 

• UNSEWERED RESIDENTIAL (BASIN 20) • HORSE AND DAIRY FARMS (BASIN 1) 

< MEDIAN VALUE LESS THAN DETECTION LIMIT 

* CONCENTRATION EXPRESSED AS MEDIAN VALUE 

NOTE: Concentrations are mean values of four samples collected seasonally from August 1996 through May 1997 

for all constituents except ammonia as N, for which two samples were collected (winter and spring 1997}. 

Values for constituents with any concentrations below detection limit are expressed as a median. 

Figure 3. Baseflow chemical quality from four small stream basins (identified in parenthesis) 

representing four different land uses within the Croton watershed in southeastern New York 

(locations are shown in figure 1 ). 

7 
DOES THE INTENSITY OF RESIDENTIAL DEVELOPMENT AFFECT THE CHEMICAL QUALITY 

OF BASEFLOW IN A PREDICTABLE WAY? 

Residential development is the 

most common form of development 

within the Croton watershed, and 

domestic wastewater is most 

commonly disposed of through 

individual septic systems. The 

intensity of the main solutegenerating activities associated with 

unsewered residential development 

(road-salt application and domesticwastewater disposal) was estimated 

and compared with baseflow chemical 

quality through simple statistical 

procedures. The intensity of 

unsewered residential development in 

each basin was estimated by 

calculating ( 1) the density of roads 

per square mile along with estimation 

of annual road-salt application per 

square mile, and (2) the density of 

housing (septic systems) per square 

mile. The responses of baseflow 

Quantifying Road-Salt 

Application 

The intensity of road-salt 

application in each sampled basin 

was quantified as an annual 

application rate (AAR), in tons per 

square mile of basin. Basin areas 

' were determined from topographic 

maps, and the number of road 

miles within each basin was obtained 

from geographic information system 

(GIS) road coverages modified from 

Linsey and others (1999). 

Adjustments were made to both data 

sets after field verification. Values 

for each basin are given in table 

1. The annual average amount of 

road salt purchased, and the number 

of road miles salted were obtained 

through telephone interviews with 

town, county, and State officials. 

8 

water-quality constituents to the 

intensity of road-salt application or 

domestic wastewater disposal varies 

from marked to no discernible effect; 

these responses, with emphasis on 

chloride and nitrate, are discussed in 

the following sections. 

Land-use-percentage data for each 

basin were not used to evaluate effects 

of land use on base flow chemical 

quality because this data set is limited 

to two broadly defined housingdensity categories that preclude 

evaluation of unsewered residential 

development as a continuum. 

Furthermore, residential land-use 

percentages are derived from 

land-cover data of limited resolution 

(30-m by 30-m pixels) and have a 

lower limit of 30 percent constructed 

area per pixel for designation as 

residential land use (U. S. Geological 

Annual application rates per lane of 

town roads were virtually the same 

as those for county and State roads, 

including the Taconic Parkway (fig. 

1), but the rate per lane for Interstate 

84 (fig. 1) was 3 to 4 times higher 

than for the other road types. 

The road classifications, and their 

respective application rates, are 

given in the table below. 

The AAR for each basin was 

calculated as the sum of the roadsalt application amounts associated 

Survey, 1998). The result is that 

some areas of low-density residential 

development are not recognized and 

are erroneously assigned to nonresidential land-use categories. 

Road-Salt Application 

Deicing salts applied to roads 

during the winter are a primary 

source of solutes to ground water 

in the study area. Sodium chloride 

in solid form is the most commonly 

used deicing salt, but calcium- and 

magnesium-chloride salts (liquid and 

solid forms) are used in some areas. 

The liquid salts provide immediate 

deicing action upon application to 

roads and are generally applied as an 

additive on slower and longer acting 

sodium chloride and sand mixtures. 

with each road category divided 

by the basin area. Road-salt 

application amounts by road 

category are equal to total road miles 

within the basin multiplied by the 

percentage of road miles represented 

by the given road category, 

multiplied by the corresponding 

application rate listed above. 

AARs for the 33 basins 

ranged from 0 to 980 (tons/ 

mi2 )/yr (table 1 ). 

[Locations of four-lane roads are shown in fig. 1.] 

Road-salt-application rate, in tons 

Road Category per mile of roadway per year 

Town, County, and State Roads (2lane) 37 

Taconic Parkway (4lane) 75 

Interstate 84 (4lane) 298 
A. B. 1000.---.-.-~~~----~--~~~ 

LOG[CI] = 0.950 * LOG[AAR] - 0.589 

r

2=0.86 

LOG[CI] = 0.933 (Log road density) + 0.873 

r2=0.90 

z 

0 

~ a: a: 

~w 

w t: 100 

().....J 

za: 

ow 

()C... 

WC/) 

0:2! 

-<( 

a: a: g~ I.....J 

() ::::! 10 

s:2! 

oz 

_JLL 

w 

Cf) 

<( 

[(l 

• • 

100 

10 

1 L_ __ L_~~~~L---L-~~~~~~L_~~~~ 

0.1 1 10 100 

1

10 100 1000 

ANNUAL APPLICATION RATE (AAR) OF SALT, 

IN TONS PER SQUARE MILE OF BASIN 

TWO-LANE ROAD DENSITY, IN MILES 

PER SQUARE MILE OF BASIN 

EXPLANATION 

• August 1996 • November 1996 • January 1997 + May 1997 

Figure 4. Relation of the concentration of chloride in baseflow in small streams in the Croton watershed in southeastern 

New York, August 1996 through May 1997, to: A. Annual road-salt application. B. Density of two-lane roads. 

Solid forms of sodium chloride 

and calcium chloride used in the 

Northeast are 98 and 97 percent 

pure, respectively (Granato, 1996); 

thus, chloride, sodium, and calcium 

(as well as magnesium, where 

magnesium chloride salt is used) 

are the primary solutes associated 

with road-salting activities that 

might affect the chemical character 

of baseflow in streams. Chloride 

is the best indicator of road-salt 

application because it is a primary 

component of all road-deicing 

compounds and because it is 

chemically unreactive in most 

environments. The concentrations of 

chloride in baseflow of streams 

sampled in this study ranged 

widely-from 1.8 to 280 mg/L. 

Secondary sources of chloride 

include domestic-wastewater and 

water-softening-unit discharges. 

Robertson and others ( 1991) 

documented 24 and 3 8 mg!L of 

chloride in the cores of wastewater 

plumes from two septic systems. 

Water-softening units discharge 

spent brine as well as calcium and 

magnesium ions to ground water 

through septic systems or dry wells; 

annual sodium chloride (halite) 

usage of 700 to 1,000 pounds 

per unit is commonplace. These 

treatment systems are most likely 

to be used in households that draw 

their water supply from marble 

bedrock or unconsolidated 

sediments containing marble, 

although their use for removal of 

iron is apparently widespread. 

Sodium, calcium, and magnesium 

vary in their usefulness as indicators 

of road-salt application in the study 

area, but may be more useful in 

other areas. Sodium is the secondbest indicator after chloride because 

sodium chloride is used in most 

areas. High concentrations, of 

sodium, however, may be decreased 

by exchange for calcium and 

magnesium at exchange sites on 

sediments in the unsaturated-zone 

and the shallow ground-water flow 

system. Secondary sources of 

sodium are domestic-wastewater and 

water-softening system discharges. 

Calcium and magnesium are less 

reliable indicators than sodium 

because ( 1) their use in road-salting 

within the watershed is sporadic, (2) 

weathering of marble bedrock is a 

major source locally, (3) domesticwastewater and water-softeningsystem discharges are secondary 

sources, and ( 4) their concentrations 

can be decreased through cationexchange processes. 

Log chloride concentrations in 

baseflow are plotted as a function 

of log annual application rate (AAR) 

of road salt in figure 4A which 

9 
represents all 3 3 basins and all three 

road categories (see highlighted 

box). Log chloride concentration 

also can be plotted as a function 

of log road density mi2 of basin 

for those basins with only two-lane 

roads, as described above (fig. 4B). 

Both plots indicate strong positive 

relations that provide two ways of 

predicting chloride concentrations in 

stream baseflow. The relation 

of log chloride to log AAR (fig. 

4A) is most useful in basins with 

more than one road category, and 

the relation of log chloride to log 

two-lane-road density (fig. 4B) is 

most useful in basins without major 

highways. This latter relation has the 

advantage that it does not require 

road-salt application-rate estimates 

(AAR), but rather, road mileage, 

which is relatively easy to determine. 

The seasonal variability in 

chloride concentrations at the 

sampled sites tends to increase with 

increasing chloride concentration; 

however, individual regression lines 

for each sampling period (not 

shown) are generally parallel and 

tightly clustered around the 

regression line for all samples. This 

indicates that the ground waters 

that sustain baseflows are a 

consistent source of chloride, unlike 

stormflows, which typically have 

the highest chloride concentrations 

during the winter and spring (Ku and 

Simmons, 1986). The consistency 

of chloride concentrations in ground 

water may be due to several factors. 

First, salts can be held at land 

surface or in the unsaturated zone, 

rather than being completely flushed 

downward to the water table during 

the spring, as evidenced by 

observations of salt crusts along 

some roads throughout the year. 

Thus, rain that infiltrates land surface 

at other times of year can dissolve 

this salt and transport it downward 

into the unsaturated zone or to the 

water table. Rises in the water table 

10 

in response to recharge resaturates 

portions of the unsaturated zone 

and may also dissolve stored salt. 

Peak concentrations of chloride in 

ground water may be decreased 

through dispersion during transport 

and by dilution through convergence 

with water from deeper zones (with 

potentially lower chloride 

concentrations) at discharge areas. 

Several other factors can 

potentially decrease or increase the 

concentrations of chloride in the 

baseflow of a given stream, and 

contribute to the scatter or bias seen 

in figures 4A and 4B. Application 

rates can be lower than predicted 

if narrow roads are salted in one 

pass rather than two, and application 

rates can be higher than predicted if 

unpaved roads are present, because 

they generally require more frequent 

salt applications than paved roads. 

Decreases in baseflow chloride 

mighl~~poc~d~~~m~ 

which efficient stormwater routing, 

including curbed streets and storm 

sewers, transports a larger percentage 

of road salt out of a basin during 

stormflows than in basins without 

such infrastructure. Similarly, steepsided basins may also transport a 

larger percentage of road salt in 

stormflows than gently sloped 

basins. Chloride concentrations in 

streams that drain sewered basins 

may be lower than those in 

unsewered basins because the sewers 

route domestic wastewater (and its 

chloride content) outside the basins 

to treatment plants. However, 

baseflow concentrations of chloride 

from sewered basins 31 and 29 

show little departure from the 

regression lines, indicating little 

effect from sewering. Finally, the 

proximity of roads and housing 

to the stream might influence 

baseflow chloride concentrations; 

higher chloride concentrations might 

be expected where roads and housing 

are concentrated near the stream 

because ground-water transport 

distance and dispersion of chloride 

are minimized. 

Domestic Wastewater Disposal 

Disposal of domestic wastewater 

through septic systems is an 

important source of solutes to the 

shallow ground-water system. 

Effluent plumes emanating from 

septic-system leach fields typically 

have slightly to moderately elevated 

concentrations of most major ions 

and of nitrate (Robertson and others, 

1991), as well as boron, which is 

used in detergents. Nitrate, boron, 

and sulfate are the most reliable 

indicators of domestic wastewater in 

stream baseflow; chloride, sodium, 

calcium, and magnesium are less 

reliable because they can be derived 

from multiple sources. 

The nutrients nitrogen and 

phosphorus in domestic wastewater 

are a particular water-quality 

concern because they can spur algal 

growth in streams upon discharge 

from ground water. Ammonia is 

typically the dominant nitrogen 

species in domestic wastewater, but 

upon discharge to the unsaturated 

zone from septic system leach fields, 

it is transformed to nitrate by 

microbial nitrification under aerobic 

conditions. Ammonia may persist 

in unsaturated-zone and groundwater-plume environments where 

anaerobic conditions exist, such as 

in sediments of low permeability 

(Robertson and Blowes, 1995) or 

where the unsaturated zone is thin. 

Nitrate from wastewater plumes can 

be removed through denitrification 

(converted to nitrous oxide or 

nitrogen gases and released to the 

atmosphere) before or during 

ground-water discharge to streams 

in the presence of organic matter 

(within streambed sediments, in 

wetland areas). Nitrate was selected 
for analysis in this study because 

it is the most common nutrient 

in baseflow; concentrations at the 

sampled sites ranged from less than 

0.05 to 3.2 mg/L. Phosphorus in 

the form of orthophosphate is an 

important nutrient in domestic 

wastewater, but it is largely removed 

through sorption on aquifer 

sediments or through precipitation 

of phosphate minerals. Phosphorus 

may persist, however, in ground 

water downgradient from old septic 

systems where sorption sites have 

been filled or where waters are 

oversaturated with respect to 

phosphate minerals (Robertson and 

others, 1991). 

The intensity of domestic 

wastewater disposal was estimated 

from the housing (and thus septic 

system) density (houses/mi2

) within 

each unsewered basin. Housing 

densities were estimated from 

topographic maps and then revised 

after field checks of housing location 

and basin area. Bas ins that were 

omitted from analysis included: (1) 

two influenced by horse and dairy 

farms (basins 1 and 35), (2) three 

fully or partially sewered residential 

basins (basins 29, 31, 44A), (3) two 

basins with wetlands downgradient 

of development and adjacent to 

the stream (basins 50A, 54A) 

(because nitrate concentrations could 

be affected by denitrification and, 

thus, may not be representative of 

housing density), and (4) five mostly 

undeveloped basins (8A, 12, 13, 19, 

39) with large wetland areas. 

Linear regressions of base flow 

nitrate concentrations as a function 

of unsewered housing density in 

21 basins for the four sampling 

periods provided differing positive 

relations that indicate seasonal 

variability in nitrate concentrations 

(fig. 5). Nitrate concentrations at 

most sites were highest during the 

winter and progressively decreased 

through the late fall and late spring 

to their lowest point during the 

summer. A major seasonal control 

on baseflow nitrate concentration 

at any given site appears to be 

temperature, which affects (1) algal 

production and plant growth and, 

therefore, the rate of biological 

uptake within the stream, and (2) 

the rate of microbial denitrification 

within or immediately below the 

streambed. Nitrate appears to affect 

algal populations substantially, in 

that algal (diatom) growth was most 

commonly noted in streams that 

drain basins with moderate- to 

high-density housing. Similarly, the 

greatest seasonal variations in nitrate 

concentration occurred in basins with 

moderate- to high-density housing. 

Plant uptake and microbial 

z 3 

Cf) 

<( 

z 

0 -a: t;(w 

a:t: 

1-...J 

Za: 

Ww2 

Oa.. 

ZCJ) 

0~ ()<( 

Wa: 

t;(~ CI:...J 

1-_J 

z~ 

:S:~1 

0 _J 

LL 

w 

Cf) 

<( 

co 

• 

denitrification are most likely to be 

important in low-gradient streams 

where there has been accumulation of 

bed material or where wetlands exist. 

Nitrate concentrations of baseflow 

are highest in the winter and lowest 

in the summer. The low summer 

nitrate concentrations, however, are 

accompanied by increased algal 

biomass, such as diatom populations 

on sandy streambeds, which can be 

suspended during stormflows and 

degrade water quality by increasing 

the load of organic carbon and 

nutrients moving downstream to 

the reservoirs. 

Departures from the regression 

line for the relation of base flow 

nitrate concentration to housing 

density in January 1997 (fig. 5) 

• • 

200 400 600 800 1 000 1 200 

UNSEWERED-HOUSING DENSITY, 

IN HOUSES PER SQUARE MILE 

August1996 

November 1996 

January 1997 

May 1997 

f(x) = 0.0017X+0.202 

f(x) = 0.0026X+0.056 

f(x) = 0.0029x+0.279 

f(x) = 0.0020x+0.086 

r

2 = 0.87 

r

2 = 0.86 

r

2 = 0.83 

r

2 = 0.74 

Figure 5. Relation of the concentration of nitrate in baseflow in small 

streams to the density of unsewered housing in the Croton watershed, 

New York. 

11 
indicate that other physical and 

biological factors influence baseflow 

nitrate concentrations. Such factors 

might include the length of groundwater flowpaths prior to discharge 

at the stream, basin slope, and 

aquifer permeability, all of which 

affect ground-water residence time 

and, therefore, the contact time in 

environments that favor biochemical 

transformation of nitrate. Two 

conditions that are essential for such 

transformations in the unsaturated 

zone, the shallow ground-water flow 

system, and wetlands are the 

presence of organic carbon and 

environments with little or no 

oxygen. A limiting factor for algal 

growth is the amount of sunlight 

reaching streams; thus, the degree 

of shading along streams, and the 

compass orientation and topography 

of basins together can affect algal 

productivity and nutrient uptake. 

Another factor contributing to 

departures from the relation could 

be the variable rates of lawnfertilizer application among basins, 

as described earlier. 

WHAT EFFECTS DO SEWERING AND RIPARIAN WETLANDS HAVE ON BASEFLOW 

NITRATE CONCENTRATIONS? 

The effectiveness of sanitary 

sewers and wetlands in decreasing 

baseflow nitrate concentrations are 

important watershed-management 

issues. Sewer effectiveness is the 

ability to transport domestic 

wastewater to a treatment facility 

without significant leakage to ground 

water. Assessment of sewer 

effectiveness, however, is 

complicated by 1) incomplete 

sewering in neighborhoods 

established prior to sewer 

installation, 2) other sources of 

nitrate, such as lawn fertilizers, and 

3) loss of nitrate, particularly during 

the summer months, through 

biological activity. Wetland areas 

along streams (riparian wetlands) 

and downgradient of development 

represent an additional level of 

biological activity over and above 

that indicated in well-drained basins 

with no appreciable wetland area 

(fig. 5). Baseflow nitrate 

concentration is plotted as a function 

of unsewered housing density 

during the winter and summer in 

figures 6A and 6B, respectively; 

these plots include six additional 

basins not used in figure 5. Two of 

the basins are sewered but have no 

wetland area (31, 44A is 30 percent 

sewered), two are sewered and have 

12 

wetland areas (3, 29), and two are 

unsewered with wetland areas (50A, 

54A). The scatter of data indicates 

that sewering and riparian wetlands 

downgradient of development 

decrease the concentrations of 

nitrate in baseflows in the receiving 

streams in nearly all cases. 

Sewering decreases baseflow 

nitrate concentration consistently 

throughout the year, whereas 

riparian wetland areas decrease 

baseflow nitrate concentrations 

most effectively during the 

summer months. 

The three entirely sewered basins 

(31, 29, 3), two of which (29, 3) 

also have riparian wetland areas, plot 

below the regression line in figure 

6; baseflow nitrate concentrations 

in these streams during the winter, 

when the effects of biological 

activity in wetlands are minimal, 

range between 49 and 68 percent 

lower than the values predicted for 

unsewered basins. The effectiveness 

of sewering in decreasing baseflow 

nitrate concentration can be 

constrained only in general terms 

because: 1) there are other sources 

of nitrate to ground water other 

than leakage from sewer lines and 

2) nitrate is not always conservative 

(stable) during transport to or within 

streams. Additional nitrate sources 

include application of lawn 

fertilizers and, in residential 

developments sewered after the 

initial development, domestic 

wastewater from houses still on 

septic systems and residual leachate 

from abandoned septic systems. 

Nitrate can be removed from 

ground water through 

denitrification in the presence of 

organic matter or by uptake from 

vegetation. In surface waters, 

nitrate can be taken up by wetland 

or riparian vegetation and algae. 

The only sewered basin without 

any wetland area (31) provides the 

best indication of the effectiveness 

of sewering. Sewers were installed 

after the residential development was 

established, so a small number of 

houses ( <5) may still be served 

by individual septic systems and 

thereby contribute wastewater to the 

local ground-water system. Baseflow 

nitrate concentration in this basin 

is 57 percent lower than the 

concentration predicted for an 

unsewered basin with the same 

housing density. This value represents 

a minimum effectiveness, however, 

as the nitrate contributions from 

remaining septic systems and lawn 

fertilizer usage are not accounted for. 
WINTER (JANUARY 1997) SUMMER (AUGUST 1996) 3.5 ,..,-,--,-..,....,---y---r--,-,..,-,--,-...,--,.,.-,.-,-,r-r-.,..-,..,~ 

z 

(f) 

<( 3 

z 

Q(( 

t:( ~ 2.5 

((_ 

1-_J 

Z(( 

ww 

() a_ 2 

Z(J) 

0~ ()<( 

W(( 

f-: <.!) 1.5 <(_ 

((.....l 

1-::::! 

z~ 

s ~ 1 

0 _J 

u.. 

~ 0.5 

<( 

(() 

30 PERCENT 

SEWERED 

• 

31 

4 

~~~~~~~~~~~~~ 

0 200 400 600 800 1 000 1200 0 200 400 600 800 1 000 1200 

UNSEWERED HOUSING DENSITY, IN HOUSES PER SQUARE MILE OF BASIN 

EXPLANATION 

• UNSEWERED RESIDENTIAL BASIN • UNSEWERED RESIDENTIAL BASIN WITH RIPARIAN WETLAND 

A SEWERED RESIDENTIAL BASIN • SEWERED RESIDENTIAL BASIN WITH RIPARIAN WETLAND 

Figure 6. Relation of the concentration of nitrate in the baseflow of small streams to the 

density of housing in basins with unsewered and sewered residential development, and the 

presence of riparian wetlands downgradient from residential development: A. Winter 1997. B. 

Summer 1996. [Basin locations are shown in fig. 1.] 

Baseflow nitrate concentrations 

from one partly sewered (30 percent) 

basin (44A) in figure 6 plots 41 

percent above the concentration 

predicted by the winter (January 

1997) regression line. The upper 

reaches of this basin contain large 

houses with large, well-maintained 

lawns; thus, widespread lawnfertilizer application may potentially 

contribute to the elevated 

concentration of base flow nitrate in 

this basin. 

The concentrations of nitrate in 

baseflows from two basins (54A, 

50A) with narrow riparian wetlands 

that are downgradient of unsewered 

residential development are less than 

those predicted through the 

regression equations (fig. 5). 

Baseflow nitrate concentrations 

show the greatest negative departures 

during the summer and the least 

negative departures during the winter 

(fig. 6, table 2). These 

those downgradient of development 

because these wetlands do not 

receive ground water derived from 

the developed area. 

departures are attributed 

to greater biological 

activity (denitrification, 

plant and algal uptake) 

in basins with riparian 

wetlands than in basins 

with no riparian wetland 

area. Wetlands 

up gradient of residential 

development probably 

have less effect on 

baseflow nitrate 

concentrations than 

Table 2. Baseflow nitrate concentration 

departures from concentrations predicted 

by the August 1996 and January 1997 

regression equations (in fig. 5) for streams 

with riparian wetlands downgradient of 

unsewered residential development 

Basin 

54 A 

50 A 

Departure, in milligrams per liter 

Summer Winter 

(August 1996) (January 1997) 

-0.59 -0.24 

-0.67 -0.57 

13 
SUMMARY 

Concentrations of selected 

chemical constituents in streambaseflow samples representing 

ground-water discharge from 33 

small basins within New York City's 

Croton watershed were evaluated 

in relation to land use and two 

measures of the intensity of 

unsewered residential development. 

Four samples were collected at each 

site, one during each season from 

July 1996 through May 1997, and 

analyzed for major ions, boron, and 

nutrients. Estimates of ground-water 

discharge (base flow) from two welldrained (minor wetland area) stream 

basins in the watershed indicate that 

it constitutes at least 60 percent of 

total annual streamflow. 

Baseflow concentrations of the 

selected chemical constituents were 

elevated to differing degrees relative 

to concentrations in the undeveloped 

forested (control) basin, depending 

on the predominant land use 

( unsewered residential, sewered 

residential, or agriculture [horse and 

dairy farms]) in the given basin. 

Baseflows in the undeveloped 

forested basin had the lowest 

concentrations of all detectable 

constituents. The unsewered 

residential basin was characterized 

by the highest baseflow 

concentrations of chloride and 

sodium, predominantly from winter 

road-salting, and the highest 

baseflow concentrations of nitrate, 

sulfate, and boron, primarily from 

domestic wastewater disposal 

through septic systems. The sewered 

residential basin had elevated 

baseflow concentrations of all 

constituents (especially chloride, 

sodium, and nitrate), but to a lesser 

extent (except orthophosphate) than 

the unsewered basin; the elevated 

concentration of orthophosphate 

could be derived from leaking sewer 

lines near the stream, lawn 

14 

fertilizers, or, less likely, housing 

that has not been connected to 

the sanitary sewers. The agricultural 

basin with the highest percentage 

(12 percent) of horse and dairy 

farms had the highest baseflow 

concentrations of ammonia, total 

ammonia plus organic nitrogen, and 

total phosphorus, and the second 

highest concentrations of the other 

nutrients. Concentrations of major 

ions and boron in baseflow from this 

basin were most similar to those of 

baseflow in the undeveloped forested 

basin, as a result of the low density 

of roads and septic systems. 

The intensity of unsewered 

residential development was 

quantified through estimation of 

annual road-salt-application rates 

(or road density) and housing 

(septic system) density for each 

basin. Linear regressions of nitrate 

concentration in baseflow and 

housing density in unsewered 

basins as well as chloride 

concentration in baseflow and 

annual road-salt application rate (or 

road density for basins with only 

two-lane roads) indicate that these 

measures of unsewered residential 

development can be used to predict 

effects on baseflow chemical 

quality. Chloride concentrations in 

baseflow show relatively stable 

positive linear relations with annual 

road-salt application rates and 

density of two-lane roads 

throughout the year. Baseflow 

nitrate concentrations show a 

different positive linear relation 

with unsewered housing density for 

each season; the highest nitrate 

concentrations were during the 

winter, and the lowest were during 

the summer. This seasonal variation 

reflects increased biological 

activity, such as algal uptake of 

nitrate within streams, or microbial 

denitrification within riparian 

wetlands or beneath the 

streambeds, during the summer. 

Nitrate concentrations in 

baseflow from sewered basins and 

from unsewered basins with riparian 

wetland between residential 

development and the stream were 

lower than those predicted by the 

relation of unsewered housing 

density to baseflow nitrate. Nitrate 

concentrations in baseflow from the 

two sewered basins were 49 and 68 

percent below the values predicted 

for unsewered basins. Summer 

nitrate concentrations in baseflow 

from two unsewered basins with 

riparian wetlands downgradient of 

residential development were about 

0.6 mg/L lower than those predicted 

by the regression equation. 

The results of this study indicate 

that local land use affects shallow 

ground water and that ground-water 

discharge is both an important 

source of streamflow and a control 

on surface-water quality. Nitrate 

concentrations in baseflow of small 

streams that drain unsewered basins 

can be predicted from the density 

of housing (septic systems) in the 

basins. Chloride concentrations in 

baseflow of sewered or unsewered 

basins can be predicted from annual 

road-salt application estimates or 

from the density of two-lane roads 

in the basins. Nitrate concentrations 

in baseflow from a sewered basin 

indicate that sewering is at least 

57 percent effective in preventing 

nitrate from entering baseflow. 

Additional nitrate contributions 

from leaking sewer lines, lawn 

fertilizers, and remaining septic 

systems are unknown. 

By PAUL M. REISIG 
REFERENCES CITED 

Butch, G. K., Lumia, Richard, and Murray, P.M., 1998, 

Water resources data, New York, water year 1997, 

Volume 1, eastern New York excluding Long Island: 

U.S. Geological Survey Water-Data Report NY-97-1, 

350 p. 

Granato, G. E., 1996, Deicing chemicals as a source 

of constituents in highway runoff: National Research 

Council, Transportation Research Board, Transportation research record No. 1533, p. 50-58. 

Harmon, J., Robertson, W. D., Cherry, J. A., and Zanini, 

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no. 6,p. 1105-1114. 

Ku, H. F. H. and Simmons, D. L., 1986, Effect of urban 

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basins on Long Island, New York: Water-Resources 

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LeBlanc, D. R., 1984, Sewage plume in a sand and gravel 

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Linsey, K. S., Wolcott, S. W., and Schoonmaker, N. 

B., 1999, Identification of potential water-resourcesmonitoring sites in the Croton reservoir system, southeastern New York: U. S. Geological Survey Open-File 

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ion and trace metal geochemistry of an acidic septicsystem plume in silt: Ground Water, v. 33, no. 2, p. 

275-283. 

Robertson, W. D., Cherry, J. A., and Sudicky, E. A., 1991, 

Ground-water contamination from two small septic 

systems on sand aquifers: Ground Water, v. 29, no. 1, 

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U.S. Department of Commerce, Bureau of Census, 1990, 

1990 census of population and housing: Public Law 

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U.S. Geological Survey, 1998, Digital map file of New 

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For Additional Information Contact: Copies of this report can be purchased from: 

District Chief 

U.S. Geological Survey, WRD 

425 Jordan Road 

Troy, NY 12180-8349 

(518) 285-5600 

U.S. Geological Survey 

Branch of Information Services 

Box 25286 

Denver, Co 80225-0286 

This report and additional earth science information can be found on the World Wide Web at: http//:ny.usgs.gov 

15