ORIGINAL ARTICLE

Using stable isotopes of nitrogen to study its sourceand transformation in a heavily farmed watershed

S. Gautam Æ M. Z. Iqbal

Received: 17 November 2008 / Accepted: 8 April 2009

� Springer-Verlag 2009

Abstract Twenty private wells and ten stream locations

were sampled to assess the source and fate of dissolved

nitrate in the Cedar River watershed of Iowa, USA. The

average levels of nitrate in groundwater decreased from

39.5 mg/L in May, to 38 mg/L in July, to 30 mg/L in

September. Although several surface water samples

exceeded MCL in May, most values dropped to below

20 mg/L by July and September. The decreasing N levels

were attributed to the gradual uptake of nitrate by growing

crops as well as the cyanobacterial growth in the aquatic

systems. The d15N values of dissolved nitrate in ground-

water ranged from ?0.45 to ?5.35%, whereas those in

surface water ranged from ?1.48 to ?5.16%. The results

suggested that commercial fertilizers and soil organic

nitrogen were probably mixed up in their transport path-

ways. A fertilizer-only source would provide much lower

delta values, whereas soil nitrogen would provide higher

than observed delta values. Denitrification was considered

unlikely because of the low d15N values, high nitrate

concentrations, and moderately high DO content in

groundwater. Animal wastes were not found as a possible

source of nitrate in the water. This is supported by the low

chloride concentrations and lower than 10% delta values in

the water samples. The study demonstrates that nitrogen

isotope data in coordination with the dissolved nitrate

levels and land use can be effectively used in nitrogen

source identification and its transformation studies.

Keywords Nitrogen � Fertilizers � Isotopes �Groundwater � Watershed

Introduction

The groundwater table of northern and northeastern Iowa is

affected by non-point sources nitrogen coming from cul-

tivated, row-cropped fields on well-drained soils in the

area. Many drinking water wells in these areas are charac-

terized by nitrate concentrations higher than maximum

contaminant levels (MCL) of 45 mg/L as NO3- or 10 mg/L

as NO3- N (U.S. EPA 2007). Agricultural leachate from

commercial nitrogen fertilizer and manures are the major

contributors. In the past two decades, the use of nitrogen

fertilizer in Iowa has increased drastically and now it is a

major threat to groundwater. As nitrate builds up in the

aquifer over time, a part of it leaves the system with base

flow discharge into the nearby streams (Iqbal 2002). The

median discharge-weighted nitrate concentrations also are

significantly greater in Eastern Iowa streams than in

streams sampled elsewhere in the Corn Belt as well as the

Northern Great Plains ecoregion (Omernik 2000). Nitrate-

nitrogen averages 5.1 mg/L in groundwater compared to

11 mg/L in surface water. In one study conducted in the

Cedar River basin, 41% of the wells tested for nitrate

exceeded the MCL (Iqbal 1996).

Nitrogen compounds are primarily a water quality con-

cern because they contribute to aquatic plant growth,

eutrophication, and toxicity. Increased nitrate concentration

has negative impacts on human health. A high concentra-

tion of nitrate in drinking water causes methemoglobine-

mia, or blue baby syndrome, because it interferes with the

oxygen-carrying capacity of blood. Exposure to nitrates has

been studied in a number of illnesses having long latency,

S. Gautam

Biological Systems Engineering Department,

Virginia Tech, Blacksburg, VA 24061, USA

M. Z. Iqbal (&)

Earth Science Department, University of Northern Iowa,

Cedar Falls, IA 50614, USA

e-mail: m.iqbal@uni.edu

123

Environ Earth Sci

DOI 10.1007/s12665-009-0165-7

including incidences of gastric and esophageal cancers and

the risk of non-Hodgkin’s Lymphoma (NHL) (Forman

1989; Ward et al. 1996).

Higher nitrate levels are found in drinking water wells

all over the Cedar River basin. In one study, nine out of 20

private wells sampled had nitrate concentrations exceeding

the U.S. EPA recommended MCL of 45 mg/L (as NO3-)

for drinking purposes (Iqbal 2000). Major forms of N that

are applied to increase crop productivity include urea [CO

(NH2)2], ammonium nitrate (NH4NO3), ammonia (NH3),

and various animal manures. Although different forms of

fertilizers are applied to agricultural fields, NO3- is usually

the major form of N (other than dissolved atmospheric N2)

in agricultural recharge (Bohlke 2002).

Monitoring fate and transport of stable nitrogen isotopes

in hydrologic systems has become an efficient method of

studying water quality in the recent decades. The stable

isotopic compositions of low-mass elements such as

nitrogen are usually reported as delta (d) values in parts per

thousand (denoted as %) enrichments or depletions relative

to a standard of known composition. As nitrogen com-

pounds are chemically altered within a system, stable iso-

topes of nitrate, 15N and 14N, may undergo isotopic

fractionation. One of the nitrogen isotopes is either incor-

porated into or depleted from the system depending on the

existing geochemical environment. Therefore, nitrogen

isotopes can be used to determine major sources of nitrate

in natural systems. Stable isotope ratios are expressed as

d15N = {[(15N/14N)x/(15N/14N)AIR] - 1}91,000, where

x = sample and AIR = the reference standard gas. The

standard is atmospheric nitrogen gas.

Several studies have been conducted over the last

30 years to identify nitrate sources in groundwater and

surface water (Showers et al. 1990; Kreitler 1975; Fogg et al.

1998). Gormly and Spalding (1979) used isotopes of nitro-

gen to find the primary sources of N in groundwater of

Nebraska. They found that the corresponding d15N range of

values were ?5 to ?9% for soil nitrogen, -2 to ?7% for

commercial fertilizer, and ?10 to ?23% for livestock.

Kreitler et al. (1978) used isotopes of nitrogen in the

groundwater of Long Island, New York and found that the N

in groundwater shifted from lighter values in the eastern part

of the island where land is used predominantly for agricul-

ture, to heavier values toward New York City, where land is

used for suburban residences with septic systems or sewers.

On an average, d15N values for inorganic fertilizer, unfer-

tilized cultivated fields, and animal wastes show a shift from

low to high values such as -3 to ?2%, ?2 to ?8%, and

?10 to ?20%, respectively. The feasibility of using nitro-

gen and oxygen isotope ratios of nitrate for elucidating

sources and transformations of riverine nitrate was evalu-

ated in a comparative study of 16 watersheds in the north-

eastern USA (Mayer et al. 2002). In conjunction with

information about land use and nitrogen fluxes, d15N and

d18O values of NO3- provided important information about

sources of riverine nitrate in these studies. In many studies,

d15N has demonstrated to be an effective discriminator

between plant or commercial fertilizer-derived nitrate and

animal-derived nitrate. In a recent study, Avisar et al. (2009)

successfully used nitrogen isotopes to confirm that the levels

of nitrate in groundwater went down as a result of their

initiative to replace cesspits for waste disposals with a

central sewage system in two Israeli villages. Within several

years after the cesspit disposal was terminated, the d15N

values of groundwater dropped from around ?8% to the

natural background level. However, division between mul-

tiple human sources and animal sources are less well defined

(Fogg et al. 1998; Kendall 1998). Although d15N of nitrate

can be used to distinguish between animal and organic N or

inorganic fertilizer derived nitrate, it has not been success-

fully used alone to distinguish between subcategories of

animal-derived nitrate in groundwater. Even coupling d15N

with other isotopes, such as d18O, has not been particularly

successful for determining differences between animal

sources.

The isotopic composition of NO3- sources can be

altered by transformations, principally denitrification, that

results in fractionation of the stable isotopes, thereby lim-

iting the utility of d15N analyses. More recent studies uti-

lized both d15N and d18O to trace the progression of N

transformations and to help separate NO3- sources. In one

study stable isotope data suggested that assimilation, not

denitrification, is the process likely to be affecting N

transport in the Mississippi River during late spring to early

summer (Battaglin et al. 2001).

Hypothesis and objectives

The objective of this study was to identify the sources of

nitrate in groundwater as well as surface water by using N-

isotope technique within the rural community of the Cedar

River watershed in Iowa. Additionally, temporal variations

in nitrate levels were examined to verify the nitrogen

source indicators.

The hypothesis of this project was that chemical fertil-

izers are the major sources of nitrate contamination in the

Cedar River watershed, which can be confirmed by stable

isotope studies. It is also hypothesized that agricultural

activities are negatively impacting the area water.

Study area

The study area is located in the Black Hawk County and

the Bremer County of the Cedar River Watershed (Fig. 1).

Environ Earth Sci

123

The area is characterized by intensive agricultural activities

with corn and soybean as major crops (Kalkhoff et al.

2000). Twenty groundwater sampling sites and ten surface

water sampling sites were selected and tested from May to

September 2006. Groundwater was sampled from private

drinking water wells, including seven wells that were

drilled into the shallow alluvial aquifer and 13 wells drilled

into the bedrock aquifers. The wells in the alluvial aquifer

range in depth from 3.7 to 9.1 m and those in the bedrock

aquifer range in depth from 15.2 to 48.8 m. The bedrock

aquifers are of Silurian, Devonian, and Ordovician time,

whereas the alluvial aquifers are composed of recent

stream deposits. Eight surface water sites were in the Cedar

River and two were in its tributaries namely the Beaver

Creek and the Shell Rock River.

The study site is located in the landform region known

as the Iowan Surface (Prior 1991). The land surface usually

appears slightly inclined to gently rolling with long slopes,

low relief, and open views to the horizon. Drainage net-

works are well established, although stream gradients

Fig. 1 Study area in the Cedar

River Watershed, Iowa

Environ Earth Sci

123

usually are low and some scattered areas of poor drainage

and original wetlands occur in the region. Erosion on a

large scale is the key to the geological origins of the Iowan

surface. Earlier, before the Wisconsinan glacial events,

the region was part of the Southern Iowa Drift Plain. It

was last glaciated in Pre-Illinoian time and since then

remained exposed to various episodes of weathering and

soil development, erosion, and loess deposition. One of the

latest episodes of erosion occurred between 16,500 and

21,000 years ago during the coldest part of the Wiscons-

inan (Prior 1991).

Hydrologic systems

The Cedar River watershed has a total drainage area of

20,242 km2, of which approximately 87% is in Iowa (Iowa

Department of Environmental Quality 1976) (Fig. 2).

There are eight main tributaries that feed the Cedar River.

The average discharge of the Cedar River where it joins the

Iowa River is 136 m3/s. During rain events, overland flow

is the main constituent of stream flow, and the source of

water during dry periods is base flow (Squillance et al.

1996). On an annual basis, it is estimated that 96% of the

flow is base flow, and the remaining 4% is surface runoff

(Squillance and Engberg 1988).

Surficial alluvial deposits are restricted to Holocene-

age river valleys and commonly consist of 9.1–30.5 m of

unconsolidated sands and gravels interbedded with less

permeable silts and clays (Wahl et al. 1978). Depth to the

water table in these alluvial aquifers typically is 1.5–

6.1 m. Recharge to alluvial aquifers is primarily from

precipitation that infiltrates through overlying unsaturated

surficial materials. Besides, water enters the aquifers

along losing stream reaches. Groundwater in alluvial

aquifers discharges to gaining stream reaches, providing

base flow (Olcott 1992). Underlying or laterally adjacent

bedrock aquifers may either receive or contribute

groundwater to alluvial aquifers (Olcott 1992). The pres-

ence of permeable sands and gravels, shallow depths to

water table, and the absence of overlying confining units

make alluvial aquifers susceptible to agricultural con-

tamination from land use activities (Hoyer and Hallberg

1991).

Bedrock aquifers are another basic class of Iowa’s

groundwater resources. Bedrock consists of sedimentary

rock layers, primarily limestone and dolomite, inter-lay-

ered with shale, siltstone, and sandstone. These rocks

originated as sediments deposited in ancient seas, rivers,

and deltas that occupied Iowa between 75 million (Creta-

ceous age) and 550 million years (Cambrian age). The total

thickness of these formations ranges from 1585 m in

southwest Iowa to about 244 m in the north-east (Prior

et al. 2003). This sequence of sedimentary rocks varies

widely in its ability to store and transmit water. The study

site has two types of bedrock aquifers, the Silurian-Devo-

nian Aquifer and the Ordovician aquifer.

Land use

About 89% of the land area in the state of Iowa is in farms.

Land use is based primarily on agriculture and agriculture-

related industry. The principal crops are corn, soybeans,

oats, hay, and pasture on unirrigated lands. Iowa produces

approximately 19% of the nation’s corn for grain and about

17% of the nation’s soybeans (USDA 2006). In addition to

row-crop agriculture, Iowa produces large quantities of

hogs, cattle, sheep and poultry. The land use of the area is

greatly influenced by agricultural activities, which are

primarily associated with growing corn and soybeans.

Consequently, pollution of groundwater in the area is of

primary concern to various groups including land use

developers, water quality researchers, and multiple end-

users.

Materials and methods

Sampling methods

Twenty private wells were sampled from May through

September 2006 in Cedar Falls, Janesville, and Waverly

areas of northeast Iowa. Additionally, ten surface water

sites were sampled, including eight in the Cedar River

main channel and its tributaries, one in the Shell Rock

River, and one in the Beaver Creek. Each site was sampled

in May, July, and September. Water samples were col-

lected in High Density Polyethylene (HDPE) plastic bottles

of 120 mL capacity. At the time of collection, on-site

testing was done for dissolved oxygen (DO). After col-

lection, samples were placed in an ice cooler to transport

back to the laboratory, and then refrigerated until they were

analyzed. Some of the water samples were sent to the

Cornell Isotope Lab (COIL) in Cornell University, New

York for nitrogen isotope analysis.

Lab analysis

The concentrations of dissolved chloride, nitrate, and sul-

fate in water were determined by ion chromatography

(Dionex� Model DX-120) under suppressed conductivity.

Ion elution was accomplished by using a CO3–HCO3

solution. The flow rate was set at 1.75 mL/min. The ana-

lytical margin of error was ±0.5 mg/L.

At Cornell Isotope Lab, samples were analyzed by using

a Thermo Delta plus isotope ratio mass spectrometer that is

specifically designed for analysis of the light element

Environ Earth Sci

123

isotope ratios of C, N, O, and H. Interfaced to the machine

is a Carlo Erba NC2500 elemental analyzer. Samples were

combusted at 1,000�C in the presence of chromium and

copper oxides and reduced to N2 using copper wire at

650�C. Water was removed from the sample using a

magnesium perchlorate water trap. The N2 was then sep-

arated in a 3 m gas chromatography column and analyzed

in the isotope ratio mass spectrometer. All of this was done

in continuous flow mode (which means that ultra-high

purity helium was the carrier gas). Samples were measured

against various in-house standards, which are biannually

calibrated against international reference materials. Both

samples and standards were calibrated against the Atmo-

spheric Air reference scale.

Fig. 2 Location of the Cedar River Basin in east-central Iowa (USGS 2000)

Environ Earth Sci

123

Results and discussion

Dissolved oxygen

There was a general decrease in Dissolved Oxygen (DO) in

groundwater from the month of May to September. In

alluvial aquifers, the wells are less than 9.1 m deep. The

DO levels in these wells ranged from 4.9 to 7 mg/L in

May. In the bedrock aquifer, wells range from 15.2 to

48.8 m in depth where the observed levels of DO were 2.9–

6.9 mg/L. The DO concentrations were within the expected

range in both aquifers. The observed levels of DO in sur-

face water were generally higher in comparison to

groundwater. According to Iowa DNR water quality stan-

dards, surface water DO must be 5 mg/L or greater (Iowa

DNR 2002). The Cedar River and its tributaries, the Shell

Rock River, and the Beaver Creek, are all perennial water

bodies. The average DO concentration in these streams

ranged from 2.9 to 9.6 mg/L. The DO levels gradually

decreased in July. During this month, the water level was

very low, which prevented mixing and aeration of atmo-

spheric oxygen into the water.

Nitrate

Ten out of 20 groundwater wells exceeded MCL of 45 mg/L

in NO3- concentration during this study. In addition, eight

out of ten surface water sites exceeded the MCL.

Nitrate concentration ranged from below detection at

site 18 (depth 38.1 m) and site 20 (depth 22.9 m) to

75.5 mg/L at site 14 (depth 36.6 m). Refer to Fig. 1 for site

locations. Sampling site 20 is purely a residential area with

no agricultural activities around it. Site 14 has barnyards,

horse farming, lawn and agricultural activities in the area,

which is likely to have contributed to the high concentra-

tion of nitrate in groundwater. The fields in the area receive

direct application of chemical fertilizers in April. The

recorded averages of dissolved nitrate in groundwater were

39.47 mg/L in May, 38 mg/L in July, and 30 mg/L in

September. The decreasing trend is attributed to the grad-

ual uptake of nitrate by growing crops as well as the

cyanobacterial growth in the aquatic systems.

Although there was no correlation between nitrate

concentration in groundwater and well depths, nitrate lev-

els were generally higher in the bedrock aquifer than in the

alluvial aquifer (Figs. 3, 4). In fact, the majority of samples

in the bedrock aquifer exceeded MCL during the study

period. Rapid preferential movement of agricultural wastes

from the fields through the fractured carbonate bedrock

may have contributed to this high level of nitrate in the

deeper wells. Frequently, irregular fracture flow is the

cause of a groundwater distribution that is characterized by

pockets of high levels of dissolved nitrate.

High nitrate levels were observed in surface water in

May, which considerably decreased in July and September.

In May, the highest concentration was 58 mg/L in site 4

(Fig. 5). Most samples in July and September had con-

centrations below 20 mg/L and all the samples that

exceeded MCL were in May, suggesting that agricultural

runoff generated by early summer rains served as the pri-

mary source of nitrate in the streams.

Low levels of nitrate were observed in a tributary stream

(Beaver Creek) during the study period (Fig. 5, site 6). DO

content was also low in the Beaver Creek, which might

suggest some transformation of nitrogen through denitrifi-

cation. But, in other surface water samples, the differences

in nitrate levels between the upstream sites (site 5, site 7,

site 8, site 9, and site 10) and the downstream sites (site 1,

site 2, site 3, and site 4) were insignificant (Figs. 1, 5).

0

10

20

30

40

50

60

70

3.7 5.2 6.1 7.3 7.6 7.6 9.1

Well Depth (m)

mg

/L

Nitrate_May

Nitrate_July

Nitrate_Sept

Fig. 3 Concentrations of dissolved nitrate in the alluvial aquifer

0

10

20

30

40

50

60

70

80

15.2

22.9

27.4

36.6

42.7

48.8

48.8

Well Depth (m)

mg

/L

Nitrate_May

Nitrate_JulyNitrate_Sept

Fig. 4 Concentrations of dissolved nitrate in the bedrock aquifer

Environ Earth Sci

123

Also, the DO contents were fairly high in these sites,

suggesting that denitrification was probably not an impor-

tant N transformation process occurring in surface water.

Isotope analysis

The study area is dominated by agricultural activities, with

the majority of land cover being corn, soybean, and forest.

These are the sources that have high potential to contam-

inate groundwater in the area. The observed d15N values in

groundwater ranged from ?0.45% in May to ?5.35% in

September. The range of d15N values indicates inorganic

chemical fertilizers and soil organic nitrogen as the

potential sources of nitrate in groundwater. These fertiliz-

ers have very low original d15N values, which reflect the

characteristics of source nitrogen (d15N: -4 to ?4%).

However, some fertilizers can have d15N ranges from -8

to ?7% (Hubner 1986). Consequently, information on land

use practices must be accurately incorporated while

applying isotope techniques in source determination of

nitrogen.

The isotopic composition of inorganic N in groundwater

is not only controlled by its origin but may also be influ-

enced by microbial and physical processes such as ion

exchange, nitrification, and denitrification (Mariotti et al.

1981). A major difficulty associated with the use of d15N to

assess the origins of nitrate is its inability to distinguish

between conservative and non-conservative fractionation

of isotopes resulting from microbial processes. For exam-

ple, loss of nitrate through denitrification can result in a

marked enrichment in 15N content of the remaining nitrate

(Mariotti et al. 1981). But, enrichment in 14N content of the

nitrate occurs during nitrification (Delwiche and Steyn

1970). In the area samples, d15N values remain low even

when the levels of dissolved nitrate in groundwater go up

or remain unchanged. In addition, there is no marked

depletion in DO content of the groundwater (Fig. 6). The

maximum d15N value in these samples is ?5.35%, which

indicates that either fertilizer or soil nitrate is the primary

source of N in groundwater.

In May, the maximum d15N value in groundwater was

?4.15% at site 17 (well depth 48.8 m). The observed

minimum in the area was ?0.45 % at site 16 (well depth:

7.3 m). The average d15N value in May was ?2.91%(Fig. 7). In July, the maximum d15N value of ?4.35% and

the minimum of ?1.35% were observed, the average being

?3.20 %. In September, the d15N values of groundwater

further increased to an average of ?3.91% (max ?5.35%;

min ?1.65%). In May, d15N value exceeded ?4% in four

out of 20 sites. Besides, five sites in July and 11 sites in

September recorded d15N values exceeding ?4%. The

general isotopic shift to higher d15N values from May to

September could be attributed to a variety of reasons

(Fig. 7). One could be the mineralization of soil nitrate

where organic nitrogen is oxidized to nitrate. Many

investigators have reported the resulting d15N values to be

greater than ?5% where soil mineralization is the major

transformation process. Clearly, the delta values of

groundwater obtained in this study are mostly below ?5%.

One way to explain the data is to interpret them by a

mixing model. It is possible that N from soil minerals and

fertilizers are mixed together at some point in their trans-

port pathways. This would place the final delta values

somewhere between the expected values for the two dif-

ferent sources. Given the land use and the observed data,

this may well have been the primary mechanism behind the

isotopic make up of groundwater in the area. Nitrification

alone could not have been the mechanism because it

generally gives the samples delta values that are much

lower than those observed in the study area. Another pos-

sible mechanism of isotopic transformation is ammonia

0

10

20

30

40

50

60

70

1 2 3 4 5 6 7 8 9 10Site

mg

/L

Nitrate_May

Nitrate_July

Nitrate_Sept

Fig. 5 Concentrations of dissolved nitrate in surface water

0

10

20

30

40

50

60

70

1.86

2.50

2.93

3.27

3.35

3.47

3.51

3.64

3.96

4.15

δ15N

mg

/L

DO Nitrate

Fig. 6 Change in DO and nitrate concentrations with increasing d15N

values in groundwater. Data shown are mean values

Environ Earth Sci

123

volatilization in samples. If ammonia volatilization occurs,

the residual nitrate becomes heavier than the fertilizer

nitrogen (Kreitler et al. 1978). Most of the fertilizers

applied to the area are ammonium nitrate and N–P–K

combinations, which are not expected to undergo signifi-

cant volatilization under adequate field moisture and a

stable soil pH. The d15N values of groundwater in the area

do not seem to vary with well depths. No such trends were

found by plotting d15N values in groundwater against well

depths (R2 = 0.066). Also, there were no major differences

in the process of fractionation between the alluvial aquifers

and the bedrock aquifer.

In surface water, transformation processes like assimi-

lation and denitrification can alter the isotopic composition

of source nitrogen. In fact, transformation of N can take

place in the soil zone, the unsaturated zone, the saturated

zone, or in the stream. This process makes it highly difficult

to distinguish between nitrate sources in streams by using

d15N values. A large range of delta values (-27 to 0%) has

been observed for assimilation of nitrate and ammonia by

algae in aquatic systems (Fogel and Cifuentes 1993).

Denitrification, which is the reduction of nitrate to nitrogen,

enriches 15N in the residual nitrate. In-stream loss of N with

higher d15N can be stronger evidence of denitrification of

nitrate in shallow aquatic systems (Clark and Fritz 1997). In

addition, volatilization of ammonia can cause fraction-

ation in surface water. The volatilization enriches the 15N

in residual solution. The enriched ammonium in solution is

then rapidly nitrified to 15N-enriched nitrate (Kreitler 1975).

The loss of nitrogen in streams by any of these transfor-

mation processes can lead to lower nitrate concentrations,

loss of N mass (to the atmosphere), and a shift toward

heavier d15N values between upstream and downstream

measuring points. In this study, d15N values in stream

samples ranged from ?1.48 to ?5.16%, which suggests

multiple sources of N and subsequent transformation in the

aquatic systems. The observed averages were ?3.33% in

May (range ?1.48 to ?4.78%), ?3.56% in July (range

?2.05 to ?4.82%), and ?4.26% in September (range

?3.09 to ?5.16%). The d15N values in the surface water

are generally in higher ranges compared to the groundwater.

Because DO content in surface water is high along with

high levels of nitrate, the water is probably not subject to

denitrification. It is particularly important to note that with

increasing d15N values, nitrate and DO levels do not go

down, which rules out denitrification as an important pro-

cess in the stream water (Fig. 8). Only four out of ten sites

recorded d15N values exceeding ?4% after fertilization of

fields in May. Low nitrate levels and DO content with lower

d15N values in Beaver Creek indicate some kind of in-

stream transformation. Chemical fertilizer may not be the

sole source of N here. Nevertheless, four sites in July and

seven sites in September exceeded d15N value of ?4%.

Higher d15N values that are outside the range of fertilizers

suggest that there were some other factors responsible for

the isotopic fractionation in the samples. If nitrified soil

organic nitrogen were the primary source of nitrate to sur-

face water, one would expect a marked increase in nitrate

concentration in the water body receiving runoff from

undeveloped land. Therefore, stable nitrate concentrations

with slightly increased d15N values indicate that soil min-

eralization was not the predominant source of N to surface

water in the area. Instead, like groundwater, mixing of N

from soil minerals and fertilizers may have taken place

during transport. A general isotopic shift to higher d15N

values was observed in surface water from May to Sep-

tember (Fig. 9). This may be the result of the perceived

mixing of N from the above two sources. Such a shift to

heavier isotope can be expected in late summer when the

accumulated mass of organic N had been adequately

exposed to the process of oxidation to produce nitrate.

Thus, isotope data can serve as a characteristic of source

N if the technique is applied in conjunction with sufficient

00.5

11.5

2

2.53

3.54

4.5

May

Months

SeptemberJuly

Mean δ15N (‰)

Fig. 7 Temporal change in isotopic composition of groundwater

during the study period

0

10

20

30

40

50

60

2.61

3.13

3.47

3.56

3.70

3.71

4.00

4.10

4.24

4.62

δ15N (‰)

mg

/L

DO Nitrate

Fig. 8 Change in DO and nitrate concentrations with increasing d15N

values in surface water. Data shown are mean values

Environ Earth Sci

123

knowledge of land use practices in the area. The method is

effective even when nitrate concentrations vary consider-

ably. The isotopic analysis of water samples from the

current study demonstrates that inorganic fertilizers along

with some overlapping sources of organic nitrogen provide

dissolved nitrate in the area’s groundwater. Animal and

human wastes were not detected as the potential contami-

nant in any samples. Similarly, d15N values were not high

enough to indicate denitrification as an important process

of fractionation. High DO and stable nitrate levels support

this idea.

Summary and conclusion

In this study, groundwater from 20 private wells and sur-

face water from ten stream locations were sampled. Of

those, ten groundwater and eight surface water samples had

dissolved nitrate that exceeded MCL, most of them in early

summer. Nitrate concentrations ranged from below detec-

tion to 75.5 mg/L in groundwater, and from below detec-

tion to 58 mg/L in surface water. However, nitrate

concentrations in surface water decreased dramatically by

late summer because most agricultural runoff occurred

during the rainfall events of May and June. Nitrate con-

centrations also varied with well depths. Levels were

higher in the bedrock aquifers than the alluvial aquifers.

This may be due to the presence of residual nitrate in the

bedrock from previous years and its slower response to

rainfall events.

The analysis of d15N of dissolved nitrate in groundwater

and surface water indicated commercial fertilizers and

organic nitrate as the possible sources of N in the area.

However, in-stream transformation of nitrogen made such

interpretations somewhat difficult for the surface water.

The observed d15N values in groundwater ranged from

?0.45 to ?5.35%. Besides, the DO was high and the

nitrate concentrations were relatively stable, suggesting

that denitrification was unlikely in the area’s groundwater.

The observed d15N values also indicate that there was no

nitrification in the soil, which generally releases very light

isotopes. Besides, no correlation was found between d15N

values and the well depths.

The d15N values in surface water ranged from ?1.48 to

?5.16%, which again suggests inorganic fertilizers and

soil mineral nitrate as the possible sources of N. As indi-

cated by the d15N values, nitrogen from animal and human

wastes was not present in any sample. This is further

supported by the low chloride concentrations in the water.

In this study, the d15N distribution in water confirms the

extensive use of fertilizers in the area. Best Management

Practices (BMP) in agriculture is the only way to reduce

the over-application of inorganic fertilizers, thereby low-

ering the concentrations of dissolved nitrate. Since no

single set of BMP applies to all situations, multiple mea-

sures should be taken to address the problem. In addition,

use of manure as a nutrient source should be widely pro-

moted to decrease the use of inorganic fertilizers. Finally,

the study shows that agricultural activities are negatively

impacting the water quality of the area and that stable

isotope technique can be quite useful in both source iden-

tification and nitrogen transformation studies.

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