Deep Nitrate in an Alluvial Valley: Potential Mechanisms for Transport
By Janae Wallace and J. Lucy Jordan
Goshen Valley, home to the agricultural communities of Goshen, Genola, and Elberta, occupies the southwestern boundary of Utah Lake–Utah’s largest freshwater lake. Like other Wasatch Front valleys to the north, Goshen Valley was once inundated by ancient lake systems, including Lake Bonneville, which eventually filled the basin with clay, silt, and sand hundreds of feet thick. These deposits now host an underground aquifer supplying water to these communities.
Many water users also rely on surface water from canals, streams, and Utah Lake to irrigate; eventually a portion of that irrigation water recharges the aquifer. However, the communities rely solely on groundwater to quench their thirst. Water can contain a variety of dissolved chemical constituents that are derived from both natural and human-related sources. Dissolved elements such as calcium, sodium, and iron occur naturally in groundwater, but sometimes constituents such as mercury or nitrate can be present, and if in high concentrations, can be a health hazard.
If water from a public water-supply system contains any constituent that exceeds the U.S. Environmental Protection Agency’s (EPA) water-quality standards, it is deemed unfit for consumption and taken off line, which can be quite costly. Water from several deep wells in Goshen Valley has high nitrate concentrations, which is a unique problem since nitrate contamination is typically associated with shallow wells located near common surficial sources of nitrate.
Aquifers in agricultural valleys typically have groundwater with measurable nitrate concentrations. Why does nitrate matter? Nitrate is considered a health risk by the EPA when concentrations, measured as nitrogen, exceed 10 milligrams per liter (mg/L). Under aerobic (high oxygen) conditions, ammonium from septic-tank effluent or animal manure can convert to nitrate, contaminate groundwater, and pose potential health risks to humans.
High nitrate levels in groundwater in Goshen Valley have been documented in a handful of deep, alluvial (>150 feet) wells or in bedrock wells (a rarity). For example, Elberta’s original public-supply well (359 feet deep) located near the town center was taken off line due to elevated nitrate concentrations (up to 44 mg/L) in 1975. High nitrate levels in deep groundwater are uncommon in other, similar agricultural settings in Utah. So, why is Goshen Valley different?
Nitrate is mostly associated with surficial processes and conditions and is sourced from irrigated lands (fertilizer), concentrated animal operations (manure), septic tanks, organic soil, and/or precipitation. A less common source is bedrock.
But, how does nitrate reach the subsurface aquifer? Several possible ways include leaching in areas with high rainfall and excessive irrigation rates; traveling down poorly constructed or improperly protected wells; and natural geologic conduits such as faults, fractures, and fissures.
In Goshen Valley, residents rely on septic-tank systems for wastewater disposal, and drain fields from these systems may also leach water and associated contaminants to the aquifer. After leaching into the subsurface, surface water with potentially elevated nitrate concentrations may mix with older groundwater within the aquifer, and one way to identify sources of nitrate is through geochemical analysis of groundwater samples.
During spring 2013 and 2014, the Utah Geological Survey (UGS) sampled water from 38 sites (mostly wells and some surface waters) in Goshen Valley to analyze for general chemistry and nitrate. Samples from several locations were analyzed for oxygen and nitrogen isotopes in nitrate (16 sites) to help us pinpoint the source of nitrate. Most of the wells sampled are completed in unconsolidated basin sediments; only five are thought to draw water from bedrock. Water levels were measured or compiled for 138 wells.
From the compiled water levels, a potentiometric map was created and shows that groundwater flows into Goshen Valley from the mountains west and south of the valley and from neighboring valleys toward Utah Lake. The water table occurs at shallower depths in the southeast, possibly due to more surface recharge sources and inter-basin groundwater flow than in the south and west. In contrast, pumping from deep irrigation wells in the western part of the valley has lowered the water table as much as 50 feet in the past 30 years and has created an area of low hydraulic gradient.
Compared to other rural, agricultural areas in Utah, Goshen Valley is unique because the majority of wells have low nitrate concentrations, whereas the deepest wells have high concentrations—one well has the highest nitrate concentration ever documented by UGS. Nitrate concentrations in our samples range from less than 0.1 to 256 mg/L, with an average nitrate concentration of ~11 mg/L, but a low median value of 0.8 mg/L due to the majority of samples (73%) having nitrate concentrations less than 2 mg/L. Ten percent of samples have concentrations that exceed the EPA water quality standard and 55% have concentrations less than 1 mg/L.
The majority of samples with nitrate concentrations at or above 1 mg/L (16 sites) were analyzed for nitrogen and oxygen isotopes in nitrate to help delineate the source(s) of the nitrate. A graph of the ratios of oxygen and nitrogen isotopes in the nitrogen molecule (δ18O and δ15N, respectively) shows Goshen samples plot in the overlapping fields of fertilizers, precipitation, and manure and septic waste, and within the range of soil nitrogen.
Generally, water having nitrate concentrations around 1 mg/L (red boxes) (except for site ID 17, a well completed in bedrock supplying water to a gravel pit on the eastern basin margin) are more depleted in δ18O than the water samples with nitrate concentrations >3 mg/L (blue diamonds). Site ID 12 is an irrigation well having the anomalously high nitrate concentration of 256 mg/L.
For wells on agriculture land dominated by feed lots, dairy operations, and homes having septic systems, the amount of δ15N in water is expected to be greater than 10 parts per thousand; most values for our samples fall between 5.1 and 7.8 parts per thousand. We suspect that nitrate with a depleted δ15N signature, as found in soil and fertilizer, is mixing with nitrate enriched in 15N, likely from a manure source. Our results indicate that dilution, not denitrification, is the dominant process of nitrate removal from the environment in Goshen Valley.
In most valley groundwater systems, water having high nitrate concentration is often correlated with wells completed in shallow unconsolidated sediments (e.g., wells less than 150 feet deep). Shallow wells are more susceptible to surficial nitrate contamination by virtue of their proximity to the source.
The highest nitrate concentrations in Goshen Valley are atypical; they occur in the deeper wells (>150 feet deep) and/or bedrock wells, which are nearly all located in the irrigated western and southwestern part of the basin. Because of a low hydraulic gradient, fresh recharge with low nitrate concentrations from mountain sources likely moves more slowly through the western part of the basin than recharge in other areas of the valley. The longer residence time of groundwater on this side of the valley (corroborated by groundwater dating methods) may contribute to the elevated nitrate concentrations in the deep wells; high-nitrate water is not being flushed out of the system.
In contrast, the east and southeast areas of the basin have a relatively higher water table and likely receive more recharge from surface water with lower nitrate concentration. Additionally, groundwater dating methods show that the eastern side of the valley is a mixture of young mountain recharge and older groundwater that may have been recharged outside of the immediate region. If nitrate contamination is occurring in the eastern side of the basin, the short-residence-time flow regime may move it out of the basin system more quickly than the western side.
Our data represent only a snapshot in time. Nitrate concentrations and nitrogen-oxygen isotopes in nitrate can vary seasonally and annually. What is not known about these high nitrate-water wells is whether the concentrations of nitrogen and oxygen isotope species have changed through time or with depth through the water column.
Further analysis of these data will help determine whether denitrification or dilution is dominant and occurring throughout the year to keep overall nitrate concentrations lower in Goshen Valley than other similar agricultural valleys in Utah. With continued population growth and installation of septic tanks in new developments, or substandard agricultural practices, the potential for nitrate contamination will increase. The UGS continues to evaluate the data to better understand the complexity of the groundwater system and the nitrate sources.
Survey Notes, v. 47 no. 3, September 2015