U.S. Department of the Interior

U.S. Geological Survey

Open-File Report 2016–1029

Prepared in cooperation with the Washington State Department of Ecology

Preliminary Characterization of Nitrogen and Phosphorus in 

Groundwater Discharging to Lake Spokane, Northeastern 

Washington, Using Stable Nitrogen Isotopes
Cover: Photograph showing Lake Spokane shoreline west of Tumtum, Washington. 

Photograph by Andrew Gendaszek, U.S. Geological Survey, August 21, 2014.
Preliminary Characterization of Nitrogen and Phosphorus 

in Groundwater Discharging to Lake Spokane, 

Northeastern Washington, Using Stable Nitrogen Isotopes 

By Andrew S. Gendaszek, Steven E. Cox, and Andrew R. Spanjer

Prepared in cooperation with the Washington State Department of Ecology

Open-File Report 2016-1029

U.S. Department of the Interior

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

SALLY JEWELL, Secretary

U.S. Geological Survey

Suzette M. Kimball, Director

U.S. Geological Survey, Reston, Virginia: 2016 

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Suggested citation:

Gendaszek, A.S., Cox, S.E., and Spanjer, A.R., 2016, Preliminary characterization of nitrogen and phosphorus in 

groundwater discharging to Lake Spokane, northeastern Washington, using stable nitrogen isotopes: U.S. 

Geological Survey Open-File Report 2016-1029, 22 p., http://dx.doi.org/10.3133/ofr20161029. 

ISSN 2331-1258 (online)
iii

Contents

Abstract ...................................................................................................................................................................... 1

Introduction................................................................................................................................................................. 2

Purpose and Scope................................................................................................................................................. 4

Description of Study Area ....................................................................................................................................... 4

Methods of Investigation............................................................................................................................................. 5

Aquatic Vegetation Sampling, N-Isotope Analysis, and Quality Control.................................................................. 5

Water-Quality Data Collection, Analysis, and Quality Control ................................................................................. 8

Results...................................................................................................................................................................... 10

δ15N of Aquatic Vegetation.................................................................................................................................... 10

Nitrogen and Phosphorus Concentrations in Shallow Groundwater...................................................................... 12

Discussion ................................................................................................................................................................ 15

Summary and Conclusions....................................................................................................................................... 16

Acknowledgments..................................................................................................................................................... 16

References Cited...................................................................................................................................................... 16

Appendix A ............................................................................................................................................................... 18

Figures

Figure 1. Map showing location of Lake Spokane study area, Stevens and Spokane Counties, 

northeastern Washington........................................................................................................................................... 3

Figure 2. Maps showing locations of Richardson’s pondweed sample collection sites in eastern 

Lake Spokane (A) and central Lake Spokane (B), northeastern Washington............................................................ 7

Figure 3. Map showing location of shallow piezometers installed at 30 locations near the shoreline of 

Lake Spokane, northeastern Washington.................................................................................................................. 8

Figure 4. Boxplot showing δ15N in Richardson’s pondweed for sampled land uses in Lake Spokane, 

northeastern Washington......................................................................................................................................... 11

Figure 5. Boxplot showing nitrate plus nitrite concentrations in groundwater for sampled land uses near the 

shoreline of Lake Spokane, northeastern Washington ............................................................................................ 13

Figure 6. Boxplot showing orthophosphate concentrations in groundwater for sampled land uses 

near the shoreline of Lake Spokane, northeastern Washington .............................................................................. 14

Tables

Table 1. Summary statistics of δ15N values in Richardson’s pondweed for sampled land uses in 

Lake Spokane, northeastern Washington................................................................................................................ 10

Table 2. Nitrogen and orthophosphate concentrations in groundwater for sampled land uses near the 

shoreline of Lake Spokane, northeastern Washington ............................................................................................ 12
iv

Conversion Factors

Inch/Pound to International System of Units

Multiply By To obtain

Length

inch (in) 2.54 centimeter (cm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Area

acre 4,047 square meter (m2)

Volume

acre-foot 1,233.48 cubic meter

International System of Units to Inch/Pound

Multiply By To obtain

Volume

liter (L) 0.2641 gallon (gal)

Datums 

Vertical coordinate information is referenced to the North American Vertical Datum of 1988 (NAVD 88).

Horizontal coordinate information is referenced to the North American Datum of 1983 (NAD 83).

Elevation, as used in this report, refers to distance above the vertical datum.
1 

Preliminary Characterization of Nitrogen and Phosphorus 

in Groundwater Discharging to Lake Spokane, 

Northeastern Washington, Using Stable Nitrogen Isotopes 

By Andrew S. Gendaszek, Stephen E. Cox, and Andrew R. Spanjer

Abstract

Lake Spokane, locally referred to as Long Lake, is a 24-mile-long section of the Spokane River 

impounded by Long Lake Dam that has, in recent decades, experienced water-quality problems 

associated with eutrophication. Consumption of oxygen by the decomposition of aquatic plants that 

have proliferated because of high nutrient concentrations has led to seasonally low dissolved oxygen 

concentrations in the lake. Of nitrogen and phosphorus, the two primary nutrients necessary for aquatic 

vegetation growth, phosphorus was previously identified as the limiting nutrient that regulates the 

growth of aquatic plants and, thus, dissolved oxygen concentrations in Lake Spokane. Phosphorus is 

delivered to Lake Spokane from municipal and industrial point-source inputs to the Spokane River 

upstream of Lake Spokane, but is also conveyed by groundwater and surface water from nonpointsources including septic tanks, agricultural fields, and wildlife. In response, the Washington State 

Department of Ecology listed Lake Spokane on the 303(d) list of impaired water bodies for low 

dissolved oxygen concentrations and developed a Total Maximum Daily Load for phosphorus in 1992, 

which was revised in 2010 because of continuing algal blooms and water-quality concerns. 

This report evaluates the concentrations of phosphorus and nitrogen in shallow groundwater 

discharging to Lake Spokane to determine if a difference exists between nutrient concentrations in 

groundwater discharging to the lake downgradient of residential development with on-site septic 

systems and downgradient of undeveloped land without on-site septic systems. Elevated nitrogen 

isotope values (δ15N) within the roots of aquatic vegetation were used as an indicator of septic-system 

derived nitrogen. δ15N values were measured in August and September 2014 downgradient of 

residential development near the lakeshore, of residential development on 300-ft-high terraces above the 

lake, and of undeveloped land in the eastern (upper) and central (lower) parts of Lake Spokane. 

Significantly lower δ15N values were measured within aquatic vegetation downgradient of undeveloped 

land in eastern Lake Spokane relative to both near-shore and terrace residential development land uses. 

Conversely, significantly higher δ15N values were measured downgradient of undeveloped land in 

central Lake Spokane relative to the two developed land uses. These results guided the location of 

subsequent groundwater sampling in March and April 2015 from 30 shallow piezometers driven into the 

near-shore area of Lake Spokane. Nitrate plus nitrite concentrations in groundwater discharging to Lake 

Spokane downgradient of undeveloped areas were significantly lower than those measured 

downgradient of both near-shore and terrace residential development. Orthophosphate concentrations in 

groundwater were not significantly different with respect to upgradient land use. 
2 

Introduction

Lake Spokane, locally referred to as Long Lake, is formed by the impoundment of the Spokane 

River by Long Lake Dam about 10 mi northwest of Spokane, Washington, and forms the boundary 

between Stevens and Spokane Counties (fig. 1). Seasonal water-quality impairments of Lake Spokane 

related to eutrophication including algal blooms and low dissolved-oxygen concentrations have 

occurred for several decades (for example, Wagstaff and Soltero, 1982; Soltero and others, 1992). High 

nutrient loads into the lake have contributed to eutrophication resulting in high productivity of aquatic 

plants and algae, the decomposition of which by oxygen-consuming bacteria contributes to low 

dissolved oxygen concentrations. The failure of Lake Spokane to meet Washington State water-quality 

standards for dissolved oxygen resulted in its inclusion within Washington State Department of 

Ecology’s (Ecology) list of impaired waterbodies under Section 303(d) of the Clean Water Act in 1992

and in the development of a Total Maximum Daily Load (TMDL) for phosphorus in 1992, which was 

revised in 2010 owing to continuing algal blooms and water-quality concerns (Moore and Ross, 2010). 

Patmont and others (1987) evaluated the response of periphyton communities to nitrogen and 

phosphorus loading and concluded that periphyton growth in Lake Spokane was limited by phosphorus. 

Potential sources of phosphorus and nitrogen to Lake Spokane include point sources such as 

municipal wastewater-treatment plants and industrial discharges and non-point sources within the 

Spokane River watershed upstream of Lake Spokane. Non-point sources of phosphorus include eroded 

soils, plant matter, wildlife, human waste, and fertilizers. Whereas the upstream phosphorus and 

nitrogen load to Lake Spokane from point sources is relatively well documented through National 

Pollution Discharge Elimination System (NPDES) permits (Cusimano, 2004), phosphorus and nitrogen 

loading from non-point sources such as onsite septic systems (OSS) is not well known. The present 

study partly addressed this uncertainty by first determining locations along the shoreline where the 

nitrogen isotope ratio (δ15N) value of rooted aquatic vegetation suggests that discharging groundwater 

contains the effluent of OSS wastewater and then determining if there is a statistical difference between 

nutrient concentrations of groundwater discharging downgradient of developed land and undeveloped 

land without OSS. Because rates of groundwater exchange into Lake Spokane were not measured, 

fluxes of phosphorus and nitrogen into Lake Spokane were not estimated. 
3 

Figure 1. Map showing location of Lake Spokane study area, Stevens and Spokane Counties, northeastern 

Washington.
4 

Purpose and Scope

The purpose of this report is to evaluate the difference in the nutrient concentration of 

groundwater discharging into Lake Spokane downgradient of developed areas containing OSS and 

undeveloped areas that do not contain OSS. Although nitrogen moves readily through shallow aquifers, 

these data will help determine if aquifer materials are saturated with respect to phosphorus, which 

readily absorbs to sediments in such a way that additional phosphorus is transmitted through the aquifer 

to Lake Spokane. The report includes a discussion of (1) δ15N of rooted aquatic plants to determine 

locations where groundwater discharging into Lake Spokane contains components of OSS effluent and 

(2) an evaluation of nitrogen and phosphorus concentrations of groundwater discharging into Lake 

Spokane downgradient of developed areas containing OSS and undeveloped areas that do not contain 

OSS. Water-quality samples were collected in spring 2015, and δ15N samples were collected in summer 

2014. 

Description of Study Area

The impoundment of the Spokane River at Long Lake Dam forms a 24-mi-long, 5,000-acre 

reservoir called Lake Spokane that has 243,000 acre-ft of water and 54 mi of shoreline (fig. 1). 

Upstream of Nine Mile Dam, which is immediately upstream of Lake Spokane, the Spokane River 

drains 5,220 mi2

 of northeastern Washington and northern Idaho, including the city of Spokane. Several 

unregulated tributaries flow into Lake Spokane, the largest of which is the Little Spokane River, which 

enters Lake Spokane 1.75 mi downstream of Nine Mile Dam. 

The predominant land uses surrounding Lake Spokane are undeveloped Ponderosa Pine forest, 

agriculture, and rural residential development within three unincorporated communities including Nine 

Mile Falls, Suncrest, and Tum Tum, where the 2010 population was about 8,000 (U.S. Census Bureau, 

2015). Residential development near Lake Spokane occurs along the shoreline and on 300-ft-high 

terraces. Sewage from houses and businesses is treated by OSS throughout the area surrounding Lake 

Spokane. Shallow groundwater primarily flows through unconsolidated Pleistocene outburst flood 

sediments with dominant groundwater-flow directions perpendicular to the Lake Spokane shoreline 

(Soltero and others, 1992). 

Discharge of the Spokane River is regulated at Long Lake Dam, which was completed in 1915, 

for hydroelectric power generation by Avista Utilities. Mean monthly discharge measured from water 

years 1892 to 2014 at the Spokane River at Spokane, Washington (U.S. Geological Survey [USGS] 

streamflow-gaging station 12422500) ranges from a high of 17,700 ft3

/s in May during snowmelt to a 

low of 1,690 ft3

/s in August during summer baseflow. The pool elevation of Lake Spokane is held at 

approximately 1,535 ft above sea level for most of the year except for short periods of 10-ft drawdown 

during the winter intended to manage pervasive aquatic plants by freezing them in shallow areas of the 

lake. Lake Spokane is shallowest at the outlet of Nine Mile Dam and becomes increasingly deeper 

towards Long Lake Dam, where maximum water depths approach 180 ft. 
5 

Methods of Investigation

Field data were collected in two phases during summer 2014 and spring 2015 to first identify 

where groundwater containing septic-system effluent discharges into Lake Spokane and then to measure 

the concentration of nutrients in near-shore groundwater discharging to Lake Spokane downgradient of 

developed and undeveloped land. During the first phase in August and September 2014, Richardson’s 

pondweed (Potamogeton richardsoni) was collected from 84 locations in Lake Spokane and analyzed 

for nitrogen isotope concentrations at the University of California–Davis, Stable Isotope Facility. 

During the second phase in March and April 2015, shallow piezometers were installed at 30 locations in 

the nearshore of Lake Spokane. Groundwater was sampled from these piezometers and analyzed for 

nutrient concentrations at the USGS National Water Quality Laboratory in Lakewood, Colorado. 

Aquatic Vegetation Sampling, N-Isotope Analysis, and Quality Control

Elevated values of δ15N in plant tissues can be indicative of nitrogen from anthropogenic sources 

like OSS (Cole and others, 2005). The relative abundance of the stable nitrogen isotopes 15N and 14N 

within aquatic plants that do not fix nitrogen from the atmosphere is primarily determined by the 

isotopic composition of water that they uptake through their roots, leaves, and shoots. The ratio of 15N 

to 14N is commonly expressed as δ15N (‰) = [(Rsample – Rreference)/Rreference] × 1,000, where R is 15N/14N 

and reference is atmospheric N2. Reporting units are parts per thousand (‰). Fractionation of the 

common stable nitrogen isotope, 14N, and a rare stable nitrogen isotope, 15N, occurs in the environment 

through biogeochemical processes including nitrification, denitrification, and N2 fixation. Nitrate within

the solid waste of humans, who occupy a higher trophic level, becomes enriched in 15N (that is, 

isotopically heavier) due to metabolic processes, which may be further enriched within septic systems 

due to denitrification resulting in wastewater with δ15N values between +10 and +30‰ (Finlay and 

Kendall, 2007). As a result, the δ15N value of nitrate within septic wastewater becomes distinguishable 

from groundwater with nitrate that is derived from atmospheric deposition, which has an isotopically 

lighter value of between -8 and +3‰ (Peterson and Fry, 1987; Kendall, 1998; Kendall and others, 

2007).
6 

Richardson’s pondweed was selected for δ15N analysis because it is rooted in the lakebed and 

occurs prolifically throughout Lake Spokane. Limiting sampling of aquatic vegetation for δ15N analysis 

to a single species prevented complications in the interpretations of δ15N values due to differential 

nitrogen isotope fractionation between plant species. Whole plants of Richardson’s pondweed were 

collected by hand in shallow (about 3- to 8-ft deep) water near the shoreline of Lake Spokane. In August 

and September 2014, a total of 84 sites were sampled (fig. 2) and site locations were distributed 

downgradient of near-shore residential development (NSRD; n = 21), terrace residential development 

(TRD; n = 31), undeveloped land in eastern Lake Spokane (EUND; n = 12), and undeveloped land in 

central Lake Spokane (CUND; n = 20). Whole plant samples were refrigerated and transported to the 

USGS Washington Water Science Center Laboratory in Tacoma, Washington, where roots were 

separated, cleaned with deionized water, weighed with an analytical balance to ±0.0001 g, and placed in 

a drying oven at 60 °C for a minimum of 24 h. δ15N analyses were limited to roots to increase the 

likelihood that the nitrogen sampled was derived from the uptake of pore water beneath the lakebed. 

Samples were considered dry when decreases in weight were less than 1 percent per hour measured 

using an analytical balance. Dried material was ground using a Teflon® tissue grinder and glass mortar 

and pestle. Five to 6 g of dried Richardson’s pondweed roots from each site were packaged in 

aluminum foil for shipment to University of California–Davis, Stable Isotope Facility for nitrogen 

isotope analysis by Elemental Analysis–Isotope Ratio Mass Spectrometry. Nine quality-control field 

replicates were analyzed to assess total variability in the data due to collection, processing, and 

laboratory analysis. Relative percent differences in the pairs of δ15N values between replicates and their 

accompanying sample ranged from 0 to 2.3 percent. Standard reference materials including glutamic 

acid and ground peach leaves with known δ15N values were analyzed in the same batches as the 

environmental samples. 

The non-parametric Wilcoxon rank-sum test was used to assess whether relative abundances of 

δ15N measured in the roots of Richardson’s pondweed downgradient of each of the land uses were

significantly different (Helsel and Hirsch, 2002). The null hypothesis that no difference between the 

populations of δ15N collected downgradient of different land uses was rejected if the corresponding 

Wilcoxon rank-sum test yielded a p-value of ≤ 0.05.
7 

Figure 2. Maps showing locations of Richardson’s pondweed sample collection sites in eastern Lake Spokane (A) 

and central Lake Spokane (B), northeastern Washington.
8 

Water-Quality Data Collection, Analysis, and Quality Control

Near-surface groundwater was collected from shallow piezometers installed at 30 locations in 

Lake Spokane in the lakebed within 20 ft of the shoreline (fig. 3). Nineteen piezometers were installed 

and sampled during March 24–26, 2015, and 11 piezometers were installed and sampled between April 

29 and 30, 2015. Each piezometer was installed using a hand-held post pounder to drive a 7-ft section of 

1-in.-diameter steel pipe with a removable steel drive point. After the pipe was driven to a depth of 

approximately 3 ft below the lakebed, a section of 3/8-in.-diameter polyethylene tubing attached to a 6-

in-long stainless steel wire mesh screen was inserted into the tubing. The 1-in.-diameter steel pipe was 

subsequently removed, and the piezometer was developed by pumping water from the piezometer using 

a peristaltic pump for about 30–60 minutes until the piezometer produced clear water.

Figure 3. Map showing location of shallow piezometers installed at 30 locations near the shoreline of Lake 

Spokane, northeastern Washington.
9 

Preparation of sampling equipment and collection of groundwater samples for water-quality 

analysis followed standard USGS practices (U.S. Geological Survey, variously dated; Wagner and 

others, 2007; Kozar and Kahle, 2013). Sample collection bottles were rinsed with de-ionized water 

followed by filtered native water before each sample was collected. Before sampling, 0.45-μm capsule

filters were conditioned by purging them with 1 L of de-ionized water. The collection and processing of 

water samples followed standard USGS practices as described in Wilde (2004) and Wilde and others 

(2004). Samples were immediately placed over ice after they were collected and kept chilled during 

shipment to the USGS National Water Quality Laboratory. 

The non-parametric Wilcoxon rank-sum test was used to assess whether nutrient concentrations 

measured in groundwater collected downgradient of each of the land uses are significantly different

(Helsel and Hirsch, 2002). The null hypothesis that no difference between the populations of nutrient 

concentrations collected downgradient of different land uses was rejected if the corresponding Wilcoxon 

rank-sum test yielded a p-value of ≤0.05. 

Quality-control samples including field blank water and sequentially collected field replicate 

samples were collected during the April and March sampling events. Field blanks were collected to 

identify the presence and magnitude of possible contamination that could originate from handling of 

equipment and supplies, preparation, collection, and handling of samples, or other factors such as 

environmental conditions, reagents, or instrumentation. Two field blank-water samples were prepared 

from reagent-grade reverse-osmosis water from the USGS Washington Water Science Center 

Laboratory that was passed through all collection and processing equipment onsite and into sample 

bottles submitted to the laboratory. In both field-water blanks, nitrate plus nitrite, orthophosphate, and 

total phosphorus concentrations were less than the reporting limits of 0.04, 0.004, and 0.10 mg/L, 

respectively. An ammonia concentration for one field-water blank (474731117315801) was 0.10 mg/L, 

while the other field-water blank was less than the 0.10 mg/L reporting limit.

Precision of analytical results are affected by many sources of variability within the field and 

laboratory environments including sample collection, processing, and analysis. To assess this 

variability, two field replicates were sequentially collected by filling two collection bottles one after the 

other at sites 474751117350901 on March 23, 2015, and 474800117352901 on March 26, 2015. 

Relative percent difference in the pairs of concentrations ranged from 0 percent for ammonia (both 

samples were less than the 0.10 mg/L reporting limit), 0.7–13.4 percent for nitrate plus nitrite, 2.4–3.7 

percent for orthophosphate, and 0 percent for total phosphorus. 

Groundwater discharge was confirmed at most sites by measuring the difference in hydraulic 

head between the lake and the underlying groundwater at 21 of the 30 piezometer locations using a 

manometer board that allows comparison of the hydraulic head of groundwater measured at the midpoint of the screen (Simonds and others, 2004). Vertical hydraulic gradients were computed by dividing 

the difference between the water levels in the surface water and the underlying groundwater by the 

depth of the midpoint of the screened interval beneath the streambed (Winter and others, 1988). 

Groundwater discharge is indicated when the water level inside the piezometer is higher than the water 

level of surface water. This condition indicates that the groundwater is moving into the surface water

and, by convention, the vertical hydraulic gradient is assigned a positive value. Conversely, a lower 

water level in the piezometer relative to surface water is assigned a negative value and indicates that 

surface water is moving into groundwater. 
10 

Results

δ15N of Aquatic Vegetation

Relative abundances of δ15N were measured from Richardson’s pondweed collected at 84 sites 

in the near-shore area of Lake Spokane downgradient of four land uses—near-shore residential 

development (NSRD), terrace residential development (TRD), undeveloped land in eastern Lake 

Spokane (EUND), and undeveloped land in central Lake Spokane (CUND). δ15N values of Richardson’s 

pondweed samples are summarized in table 1 and figure 4 and detailed in table A1. The δ15N values 

were not significantly different between Richardson’s pondweed sample collected from NSRD and TRD

land uses (Wilcoxon rank sum test; p-value 0.29). The δ15N value of Richardson’s pondweed collected 

downgradient of EUND was significantly less than that of Richardson’s pondweed collected 

downgradient of NSRD (Wilcoxon rank sum test; p-value 0.004), TRD (Wilcoxon rank sum test; pvalue <0.001), and CUND (Wilcoxon rank sum test; p-value <0.001). Conversely, the δ15N value of 

Richardson’s pondweed collected downgradient of CUND is significantly greater than that of the δ15N 

value of Richardson’s pondweed collected downgradient of NSRD (Wilcoxon rank sum test; p-value 

0.003), TRD (Wilcoxon rank sum test; p-value 0.010), and EUND (Wilcoxon rank sum test; p-value 

<0.001). 

Table 1. Summary statistics of δ15N values in Richardson’s pondweed for sampled land uses in Lake Spokane, 

northeastern Washington. 

Upgradient land use

Number of 

samples

δ15N (‰)

Median Interquartile range

Near-shore residential development (NSRD) 21 7.41 1.80

Terrace residential development (TRD) 31 7.77 1.41

Undeveloped land in eastern Lake Spokane (EUND) 12 6.07 1.64

Undeveloped land in central Lake Spokane (CUND) 20 8.59 0.78
11 

Figure 4. Boxplot showing δ15N in Richardson’s pondweed for sampled land uses in Lake Spokane, northeastern 

Washington. 
12 

Nitrogen and Phosphorus Concentrations in Shallow Groundwater

Nutrient concentrations in groundwater collected at 30 shallow piezometers near the shoreline of 

Lake Spokane during March and April 2015 are summarized in table 2 and detailed in table A2. Ten 

piezometers were located downgradient of each of the three types of land uses—NSRD, TRD, and 

EUND. Hydraulic gradient between groundwater and surface water was measured at 21 of the 30 

shallow piezometers. A positive hydraulic gradient was measured at 17 piezometers, a negative 

hydraulic gradient was measured at 3 piezometers, and a neutral hydraulic gradient was measured at 1 

piezometer (table A2). Most nitrogen concentrations measured in samples from the 30 piezometers were 

nitrate or nitrite, whereas ammonia concentrations of less than the 0.10 mg/L reporting limit were 

measured at 23 of the 30 piezometers. Median nitrate plus nitrite concentrations were 1.49 mg/L 

(interquartile range of 2.32 mg/L) and ranged from less than the 0.04 mg/L reporting limit to a 

maximum of 4.92 mg/L. Most phosphorus concentrations measured in samples from the 30 piezometers 

were orthophosphate, which ranged from 0.008 to 0.24 mg/L, with a median concentration of 0.044 

mg/L (interquartile range of 0.047 mg/L). 

Table 2. Nitrogen and orthophosphate concentrations in groundwater for sampled land uses near the shoreline of 

Lake Spokane, northeastern Washington. 

[mg/L, milligram per liter]

Upgradient land use

Number of 

samples

Nitrate plus nitrite (mg/L) Orthophosphate (mg/L) 

Median Interquartile 

range

Median Interquartile 

range

Near-shore residential development (NSRD) 10 2.39 1.01 0.044 0.019

Terrace residential development (TRD) 10 2.13 3.05 0.044 0.025

Undeveloped land in eastern Lake Spokane (EUND) 10 0.58 0.88 0.043 0.088

The relation between nutrient concentrations and upgradient land use varied. Nitrate plus nitrite 

concentrations downgradient of the EUND land use (median: 0.58 mg/L; fig. 5) were significantly less 

than concentrations downgradient of NSRD land use (median: 2.39 mg/L; Wilcoxon rank sum test, pvalue <0.001). Nitrate plus nitrite concentrations downgradient of the EUND and TRD land uses 

(median: 2.13 mg/L; Wilcoxon rank sum test, p-value 0.14) were not significantly different. 

Orthophosphate concentrations at piezometers downgradient of EUND (median: 0.043 mg/L; fig. 6) and 

either NSRD (median: 0.044 mg/L; Wilcoxon rank sum test, p-value 1) or TRD (median: 0.044 mg/L; 

Wilcoxon rank sum test, p-value 0.910) were not significantly different.
13 

Figure 5. Boxplot showing nitrate plus nitrite concentrations in groundwater for sampled land uses near the 

shoreline of Lake Spokane, northeastern Washington. 
14 

Figure 6. Boxplot showing orthophosphate concentrations in groundwater for sampled land uses near the shoreline 

of Lake Spokane, northeastern Washington. 
15 

Discussion

The indication of septic wastewater in Lake Spokane downgradient of different land uses was 

analyzed by measuring the δ15N value of Richardson’s pondweed in August and September 2014. These 

data were used to guide the location of subsequent groundwater sampling in March and April 2015. The 

δ15N value of Richardson’s pondweed, which is integrated across the spring and summer growing 

season, was enriched in δ15N (that is, isotopically heavier) in samples downgradient of all four land uses 

relative to the atmosphere (table 1). There was no statistical difference between δ15N values of 

pondweed samples collected downgradient of the NSRD and TRD land uses. However, the δ15N value 

of pondweed sampled downgradient of the NSRD and TRD land uses was significantly higher than 

pondweed sampled downgradient of EUND, but was significantly lower than pondweed sampled 

downgradient of CUND. Independent measurements of groundwater flux were not made during the 

growing season when the pondweed integrated its δ15N value; thus, the δ15N value of plants may reflect 

the δ15N value of surface water where groundwater is not discharging to the lake. In part, uptake of 

surface water by plants via pore water roots may explain the discrepancy between the δ15N values of 

pondweed sampled downgradient of EUND and CUND land uses. In the absence of independent 

groundwater-discharge data, subsequent groundwater sampling of nutrients was focused downgradient 

of EUND land use, which had a relatively lower δ15N value (that is, isotopically lighter), for comparison 

to groundwater sampled downgradient of NSRD and TRD land uses. 

Nutrient concentrations, including phosphorus and nitrogen, were measured at 30 piezometers in 

March and April 2015 downgradient of the EUND land use for comparison to groundwater sampled 

downgradient of NSRD and TRD land uses. No significant difference was measured in orthophosphate 

concentrations between sample locations downgradient of the EUND area and the two developed land 

uses (NSRD and TRD). Conversely, nitrate plus nitrite in groundwater sampled downgradient of the 

EUND land use was significantly less than nitrate plus nitrite in groundwater sampled downgradient of 

the two developed land uses (NSRD and TRD). Nitrogen, primarily in the form of nitrate, does not react 

with sediments or other aquifer materials and thus moves readily through groundwater, but may be 

attenuated by denitrifying microbes, which are present in most surficial aquifers (Chapelle, 1993). 

Relative to nitrogen, phosphorus is relatively immobile in groundwater because it sorbs onto soils and 

aquifer materials (U.S. Geological Survey, 1999). The concentration of iron oxides and clay minerals 

within aquifer materials as well as geochemical conditions within the aquifer, such as low pH, increase 

an aquifer’s sorption of phosphorus and limit the ability of phosphorus to discharge into surface-water 

bodies (Domagalski and Johnson, 2011). Continued loading of phosphorus saturates the sorption sites, 

leading to breakthrough and movement of additional phosphorus supplied to the aquifer. The lack of 

statistical difference between phosphorus concentrations measured downgradient of sampled areas with 

high densities of OSS (for example, NSRD and TRD land uses) and low densities of OSS (EUND land 

use) suggests that breakthrough conditions with respect to phosphorus do not exist in the area sampled. 

However, positive vertical hydraulic gradients suggest that groundwater does result in some nutrient 

inputs to the lake. 
16 

Summary and Conclusions

Nitrogen-isotope analyses of Richardson’s pondweed sampled downgradient of land uses with 

varying degrees of development and on-site septic system densities suggest that septic system influence 

was less for the undeveloped land in eastern Lake Spokane (EUND) than either developed land (NSRD 

and TRD) or undeveloped land in central Lake Spokane (CUND). Nitrogen concentrations in 

groundwater, as measured by nitrate plus nitrite, downgradient of the undeveloped land in eastern Lake 

Spokane (EUND) was significantly less than both of the developed land uses (NSRD and TRD). 

Orthophophate concentrations in groundwater, however, which is less mobile in groundwater because it 

sorbs readily to sediments, was not significantly different between samples collected downgradient of 

the undeveloped land in eastern Lake Spokane (EUND) and either of the developed land uses (NSRD 

and TRD). These data indicate that aquifer materials downgradient of on-site septic systems have not 

been saturated with respect to phosphorus and breakthrough conditions do not exist. Because 

phosphorus and nitrogen concentrations in groundwater were only measured in March and April 2015, 

temporal variability of these concentrations remains unknown. 

Acknowledgments

The authors would like to thank private landowners and citizens of the area surrounding Lake 

Spokane for access to sites along the shoreline of Lake Spokane. The authors would also like to thank 

members of the Lake Spokane Association, Llyn Doremus, Ted Hamlin, and Wyatt Plastino for field 

assistance. 

References Cited

Chapelle, F.H., 1993, Ground-water microbiology & geochemistry: New York, John Wiley & Sons, 

Inc., 424 p. 

Cole, M.L., Kroeger, K.D., McClelland, J.W., and Valiela, I., 2005, Macrophytes as indicators of land‐

derived wastewater—Application of a δ15N method in aquatic systems: Water Resources Research, v. 

41, W01014. [Also available at http://dx.doi.org/10.1029/2004WR003269.] 

Cusimano, B., 2004, Spokane River and Lake Spokane (Long Lake) pollutant loading assessment for 

protecting dissolved oxygen: Washington State Department of Ecology Publication 04-03-006, 102 p. 

Domagalski, J.L., and Johnson, H.M., 2011, Subsurface transport of orthophosphate in five agricultural 

watersheds, USA: Journal of Hydrology, v. 409, p.157–171. [Also available at 

http://dx.doi.org/10.1016/j.jhydrol.2011.08.014.] 

Finlay, J.C., and Kendall, C., 2007, Stable isotope tracing of temporal and spatial variability in organic 

matter sources to freshwater ecosystems, in Michener, R.H., and Lajtha, K., eds., Stable isotopes in 

ecology and environmental science (2d ed.): Oxford, Blackwell Publishing, chap. 10, p. 283–333. 

Helsel, D.R., and Hirsch, R.M., 2002, Statistical methods in water resources: U.S. Geological Survey 

Techniques of Water-Resources Investigations, book 4, chap. A3, 522 p. [Also available at 

http://pubs.usgs.gov/twri/twri4a3/.]

Kendall, C., 1998, Tracing nitrogen sources and cycling in catchments, in Kendall, C., and McDonnell, 

J.J., eds., Isotope tracers in catchment hydrology: Amsterdam, Elsevier, p. 519–576. 

Kendall, C., Elliott, E.M., and Wankel, S.D., 2007, Tracing anthropogenic inputs of nitrogen to 

ecosystems, in Michener, R.H., and Lajtha, K., eds., Stable isotopes in ecology and environmental 

science (2d ed.): Oxford, Blackwell Publishing, p. 375–449. 
17 

Kozar, M.D., and Kahle, S.C., 2013, Quality-assurance plan for groundwater activities, U.S. Geological 

Survey, Washington Water Science Center: U.S. Geological Survey Open-File Report 2013-1151, 88 

p. [Also available at http://pubs.usgs.gov/ofr/2013/1151/.] 

Moore, D.J., and Ross, J., 2010, Spokane River and Lake Spokane dissolved oxygen total maximum 

daily load—Water quality improvement report: Washington State Department of Ecology Publication 

07-10-073, 92 p. 

Patmont, C.R., Pelletier, G.J., Singleton, L., Soltero, R., Trial, W., and Welch, E., 1987, The Spokane 

River Basin—Allowable phosphorus loading: Washington State Department of Ecology Publication 

No. 87-e29, 186 p. 

Peterson, B.J., and Fry, B., 1987, Stable isotopes in ecosystem studies: Annual Review of Ecology and 

Systematics, v. 18, p. 293–320. 

Simonds, F.W., Longpré, C.I., and Justin, G.B., 2004, Groundwater system in the Chimacum Creek 

Basin and surface water/ground water interaction in Chimacum and Tarboo Creeks and the Big and 

Little Quilcene Rivers, Eastern Jefferson County, Washington: U.S. Geological Survey Scientific 

Investigations Report 2004–5058, 49 p. [Also available at http://pubs.usgs.gov/sir/2004/5058/.] 

Soltero, R.A., Sexton, L.M., Wargo, L.L., Geiger, D.D., Roberton, K.S., Bolstad, K.E., Buchanan, J.P., 

and Johnson, M.S.,1992, Assessment of nutrient loading sources and macrophyte growth in Long 

Lake (Lake Spokane), WA and the feasibility of various control measures: Cheney, Eastern 

Washington University, Departments of Biology and Geology. 

U.S. Census Bureau, 2015, Population, GIS data, Census 2000 and 2010 Tiger/Line files: U.S. Census 

Bureau database, accessed November 1, 2015, at https://www.census.gov/geo/maps-data/data/tigerdata.html. 

U.S. Geological Survey, variously dated, National field manual for the collection of water-quality data: 

U.S. Geological Survey Techniques of Water-Resources Investigations, book 9, chaps. A1–A10, 

accessed April 5, 2013, at http://water.usgs.gov/owq/FieldManual/. 

U.S. Geological Survey, 1999, The quality of our nation's waters–Nutrients and pesticides: U.S. 

Geological Survey Circular 1225, 82 p. 

Wagner, R.J., Kimbrough, R.A., and Turney, G.L., 2007, Quality-assurance plan for water-quality 

activities in the U.S. Geological Survey Washington Water Science Center: U.S. Geological Survey 

Open-File Report 2007–1307, 49 p. 

Wagstaff, W.H., and Soltero, R.A., 1982, The cause(s) of continued hypolimnetic anoxia in Lake 

Spokane, Washington following advanced wastewater treatment by the City of Spokane: Cheney, 

Eastern Washington University, Department of Biology. 

Wilde, F.D., ed., 2004, Cleaning of equipment for water sampling (ver. 2.0): U.S. Geological Survey 

Techniques of Water-Resources Investigations, book 9, chap. A3. 

Wilde, F.D., Radtke, D.B., Gibs, Jacob, and Iwatsubo, R.T., eds., 2004 with updates through 2009, 

Processing of water samples (ver. 2.2): U.S. Geological Survey Techniques of Water-Resources 

Investigations, book 9, chap. A5, April 2004, accessed November 1, 2015, at 

http://pubs.water.usgs.gov/twri9A5/. 

Winter, T.C., Harvey, J.W., Franke, O.L., and Alley, W.M., 1998, Ground water and surface water a 

single resource: U.S. Geological Survey Circular 1139, 79 p. 
18 

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19 

Appendix A. 

Table A1. δ15N values measured in Richardson’s pondweed in Lake Spokane, northeastern Washington. 

[Upgradient land use: CUND, undeveloped land in central Lake Spokane; EUND, undeveloped land in eastern Lake 

Spokane; NSRD, near-shore residential development; TRD, terrace residential development] 

U.S. Geological 

Survey

site No.

Upgradient land use Date Time δ15N (‰) 

474834117361401 NSRD 8/19/2014 1207 6.67

474835117361601 NSRD 8/19/2014 1145 6.53

474838117361801 NSRD 8/19/2014 1122 8.23

474843117362401 NSRD 8/19/2014 1231 8.17

474847117362701 NSRD 8/19/2014 1254 8.76

474853117363001 TRD 8/19/2014 1310 6.84

474859117363501 TRD 8/19/2014 1328 8.03

474826117360601 NSRD 8/19/2014 1429 7.41

474823117360301 NSRD 8/19/2014 1450 8.96

474821117360001 NSRD 8/19/2014 1502 7.19

474819117355701 NSRD 8/19/2014 1515 6.67

474814117355201 NSRD 8/19/2014 1528 6.91

475012117381701 TRD 8/20/2014 0947 6.54

475009117375201 TRD 8/20/2014 1012 5.04

475009117374901 TRD 8/20/2014 1023 7.30

475008117374801 TRD 8/20/2014 1038 7.17

475007117374501 TRD 8/20/2014 1051 7.38

475006117374401 TRD 8/20/2014 1102 8.85

475005117374301 TRD 8/20/2014 1118 7.42

475003117373901 TRD 8/20/2014 1130 8.36

475002117373801 TRD 8/20/2014 1136 7.69

474959117373401 TRD4 8/20/2014 1153 7.84

474957117373001 TRD 8/20/2014 1207 9.00

474957117372801 TRD 8/20/2014 1215 7.12

474955117372601 TRD 8/20/2014 1302 7.72

474954117372701 TRD 8/20/2014 1315 7.72

475010117375601 TRD 8/20/2014 1331 5.80
20 

U.S. Geological 

Survey

site No.

Upgradient land use Date Time δ15N (‰) 

475010117375801 TRD 8/20/2014 1342 7.98

475010117380001 TRD 8/20/2014 1347 5.57

475010117380301 TRD 8/20/2014 1356 8.02

475010117380601 TRD 8/20/2014 1411 7.50

474810117354601 NSRD 8/20/2014 1534 9.24

474807117354101 NSRD 8/20/2014 1552 8.74

474806117354001 NSRD 8/20/2014 1607 8.47

475235117420501 CUND 8/21/2014 0941 8.00

475235117420401 CUND 8/21/2014 0951 8.43

475240117420201 CUND 8/21/2014 1007 7.40

475241117420101 CUND 8/21/2014 1016 9.64

475242117420001 CUND 8/21/2014 1024 8.30

475244117415801 CUND 8/21/2014 1043 8.49

475245117415701 CUND 8/21/2014 1050 8.68

475244117415701 CUND 8/21/2014 1057 7.96

475246117415701 CUND 8/21/2014 1105 9.48

475247117415801 CUND 8/21/2014 1114 8.16

475248117415701 CUND 8/21/2014 1128 8.20

475243117413301 CUND 8/21/2014 1145 8.82

475240117413601 CUND 8/21/2014 1156 8.43

475242117413401 CUND 8/21/2014 1235 8.75

475243117413101 CUND 8/21/2014 1245 8.97

475244117413101 CUND 8/21/2014 1252 9.09

475245117413001 CUND 8/21/2014 1259 8.98

475246117412901 CUND 8/21/2014 1307 7.77

475243117413201 CUND 8/21/2014 1316 8.73

475242117413301 CUND 8/21/2014 1324 9.17

474805117353701 NSRD 8/21/2014 1538 6.69

474804117353501 NSRD 8/21/2014 1548 7.64

474803117353301 NSRD 8/21/2014 1604 7.98

474905117363701 TRD 9/16/2014 1011 8.00

474904117363701 TRD 9/16/2014 1014 7.77
21 

U.S. Geological 

Survey

site No.

Upgradient land use Date Time δ15N (‰) 

474906117363901 TRD 9/16/2014 1025 8.95

474909117363901 TRD 9/16/2014 1031 8.45

474803117340901 TRD 9/16/2014 1127 6.94

474913117364401 TRD 9/16/2014 1203 13.73

474914117364501 TRD 9/16/2014 1216 11.44

474917117364501 TRD 9/16/2014 1246 9.86

474918117364601 TRD 9/16/2014 1257 9.95

474919117364701 TRD 9/16/2014 1306 8.95

474749117350301 NSRD 9/16/2014 1305 5.68

474750117350501 NSRD 9/16/2014 1350 5.58

474751117350701 NSRD 9/16/2014 1414 9.00

474753117351301 NSRD 9/16/2014 1436 6.54

474801117352901 NSRD 9/17/2014 1020 6.21

474755117322001 EUND 9/17/2014 1117 4.46

474754117321701 EUND 9/17/2014 1140 4.87

474721117325501 EUND 9/17/2014 1248 6.93

474725117315501 EUND 9/17/2014 1304 6.31

474732117315901 EUND 9/17/2014 1408 6.65

474732117315801 EUND 9/17/2014 1418 5.84

474733117320001 EUND 9/17/2014 1428 6.57

474740117300501 EUND 9/17/2014 1439 8.35

474801117323101 EUND 9/17/2014 1518 5.15

474801117323301 EUND 9/17/2014 1530 6.93

474802117323501 EUND 9/17/2014 1548 2.94

474803117323701 EUND 9/17/2014 1602 5.71
22 

Table A2. Water-quality data measured at shallow piezometers for land uses near the shoreline of Lake Spokane, 

northeastern Washington. 

[Locations of shallow piezometers are shown in figure 3. Upgradient land use: CUND, undeveloped land in central Lake Spokane; 

EUND, undeveloped land in eastern Lake Spokane; NSRD, near-shore residential development; TRD, terrace residential development; 

BLNK, blank sample; REP, replicate sample. Hydraulic gradient: +, positive; -, negative; n.m., not measured. Additional data available 

in the U.S. Geological Survey National Water Information at http://nwis.waterdata.usgs.gov/nwis. N, nitrogen; P, phosphorus] 

USGS Site No. Upgradient 

land use Date Time Specific 

conductance

Hydraulic 

gradient

Ammonia 

(mg/L as N) 

Nitrate plus

nitrite

(mg/L as N) 

Orthophosphate

(mg/L as P) 

Total 

dissolved 

phosphorus

(mg/L as P)

474755117351901 NSRD 3/24/2015 1150 375 + 0.18 0.321 0.008 0.04

474754117351601 NSRD 3/24/2015 1220 554 n.m. <0.01 4.92 0.033 0.03

474751117350901 NSRD 3/24/2015 1400 427 n.m. <0.01 2.69 0.054 0.05

474806117353901 NSRD 3/24/2015 1540 406 + <0.01 2.69 0.078 0.07

474807117354001 NSRD 3/24/2015 1620 425 + 0.02 2.90 0.048 0.05

474849117362801 NSRD 3/25/2015 1000 619 + <0.01 2.21 0.158 0.16

474815117355401 NSRD 3/25/2015 1620 458 + <0.01 2.19 0.034 0.03

474800117352901 NSRD 3/26/2015 1650 360 n.m. <0.01 1.51 0.041 0.04

474835117361501 NSRD 4/29/2015 1350 560 + <0.01 2.56 0.046 0.04

474825117360401 NSRD 4/29/2015 1510 331 + <0.01 1.50 0.028 0.03

474859117363401 TRD 3/25/2015 1100 598 + <0.01 0.112 0.086 0.08

474902117363501 TRD 3/25/2015 1130 461 + <0.01 0.108 0.028 0.03

474907117363901 TRD 3/25/2015 1220 528 n.m. <0.01 2.78 0.027 0.02

474913117364501 TRD 3/25/2015 1240 539 - <0.01 4.55 0.042 0.04

475009117375001 TRD 3/25/2015 1400 488 - 0.05 <0.040 0.017 0.02

474958117373001 TRD 3/25/2015 1440 156 - <0.01 0.287 0.045 0.05

474747117345601 TRD 4/30/2015 1140 601 + <0.01 3.35 0.097 0.10

474747117345001 TRD 4/30/2015 1010 386 + <0.01 2.10 0.034 0.03

474909117364001 TRD 4/29/2015 1230 571 + <0.01 2.16 0.057 0.05

474919117364601 TRD 4/29/2015 1320 547 neutral <0.01 4.51 0.048 0.03

474803117323801 EUND 3/26/2015 1150 484 n.m. <0.01 0.612 0.127 0.12

474803117323601 EUND 3/26/2015 1250 397 n.m. <0.01 0.406 0.083 0.07

474802117323401 EUND 3/26/2015 1330 706 n.m. 0.20 0.050 0.018 0.01

474800117322801 EUND 3/26/2015 1410 320 n.m. 0.01 0.556 0.061 0.07

474813117330401 EUND 3/26/2015 1540 484 n.m. <0.01 0.713 0.240 0.24

474754117321801 EUND 4/30/2015 1410 421 + 0.03 <0.040 0.108 0.10

474712117320301 EUND 4/30/2015 1540 337 + 0.03 <0.040 0.008 <0.01

474731117315801 EUND 4/30/2015 1720 325 + <0.01 1.47 0.013 <0.01

474726117315501 EUND 4/30/2015 1700 300 + <0.01 1.28 0.013 <0.01

474720117315601 EUND 4/30/2015 1630 286 + <0.01 1.12 0.025 0.02

474751117350901 REP 3/24/2015 1401 427 n.m. <0.01 2.33 0.056 0.05

474800117352901 REP 3/26/2015 1651 360 n.m. <0.01 1.50 0.042 0.04

474803117323801 BLNK 3/26/2015 2300 n.m. n.m. <0.01 <0.040 <0.004 <0.01

474731117315801 BLNK 5/1/2015 2300 9.2 n.m. 0.01 <0.040 <0.004 <0.01
Publishing support provided by the U.S. Geological Survey

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For more information concerning the research in this report, contact the

 Director, Washington Water Science Center

U.S. Geological Survey

934 Broadway, Suite 300

Tacoma, Washington 98402 

http://wa.water.usgs.gov
Gendaszek and others—Nitrogen and Phosphorus in Groundwater Discharging to Lake Spokane, Northeastern Washington—Open-File Report 2016–1029

ISSN 2331-1258 (online)

http://dx.doi.org/10.3133/ofr20161029