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- Drinking Water
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- Index of Vulnerability for Elevated Nitrates in Groundwater
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Robert Black
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- Last Updated
- 1/17/2024 3:22:13 PM
This indicator estimates the likelihood of elevated nitrate concentrations in groundwater throughout Puget Sound. Elevated nitrate concentrations are typically caused by human activities and can impact drinking water and human health. The modeled results represent all areas in the Puget Sound region, including tribal and rural communities with limited data, to identify areas that are vulnerable to groundwater contamination.
Modeling results from Black et al. (2023) were used to assess how the predicted probability of finding elevated nitrates in groundwater at specific locations and depths in Puget Sound changed from 2000 to 2019. Probabilities can range from 0 to 100%. The figure shows that most of the modelled Puget Sound groundwater (about 90%) had a low probability (less than 25%) of elevated nitrate, but a slight increase in these low probabilities between 2000 and 2019. Groundwater with a moderate probability of elevated nitrates (25 to 70%) accounted for approximately 9% of the Puget Sound groundwater. Probabilities at these locations generally declined between 2000 and 2019, particularly at 150-foot deep wells. Groundwater with a high probability of elevated nitrates (more than 70%) saw a significant increasing trend in probability between 2000 and 2019, but only accounted for approximately 1% of the Puget Sound groundwater.
A detailed description of how this figure was created and the data used can be found in Black et al. (2023) and Wright et al. (2023), respectively.
Groundwater provides water to public drinking supplies for over half of the Puget Sound population. A variety of chemicals, including nitrate, can pass through the soil and potentially contaminate groundwater. This indicator identifies areas with conditions (for example land use practices, geology, and well depth) that make groundwater more vulnerable to human caused nitrate contamination.
Most groundwater in Puget Sound is not contaminated by nitrates, however, it is a concern because contaminated drinking water can cause serious health risks, especially in infants. When ingested, nitrate can cause low oxygen levels in the blood which can lead to a potentially fatal condition known as methemoglobinemia or “blue baby” disorder. For this reason, public water systems carefully monitor nitrate levels in source water and treat the water when necessary.
This indicator builds on our understanding of regional nitrate concentrations from source water monitoring by modeling the risk of elevated contamination across all areas including tribal and rural communities with limited data.
- Most of Puget Sound is at low risk for elevated nitrate concentrations in groundwater. Modeled probabilities show that for more than 75% of the Puget Sound basin area, the risk of elevated nitrate concentrations is less than 10%.
- However, relatively high probabilities are estimated for several Puget Sound lowland locations. The most vulnerable groundwater to elevated nitrates (highest probability of having nitrate concentrations equal to or greater than 2 mg/L) are located near areas of high population density or increased agricultural activity.
- Nitrate vulnerability is lower in deeper groundwater. This means that as well depth increases, the probability of elevated nitrate concentrations decreases. Nitrate contamination on ground surfaces is less likely to reach deeper groundwater due to physical or chemical processes.
- From 2000 to 2019, the highest probabilities of finding elevated nitrate concentrations have steadily increased. This is the result of more locations having the highest probabilities of elevated nitrates (greater than 70% probability) and an increase in probabilities of those areas that already have high probabilities. The areas with these increases in nitrate vulnerability occur in approximately 1% of Puget Sound groundwater but are generally located in populated or agricultural areas. In contrast, groundwater with a moderate probability of elevated nitrates (25 to 70%) generally saw decreasing trends in probabilities between 2000 and 2019.
- This indicator and the probability maps do not represent actual nitrate contamination of groundwater. Rather they help us understand where conditions occur that increase the probability of elevated nitrate concentrations.
- Monitoring Program
U.S. Geological Survey Washington Water Science Center
- Data Source
U.S. Geological Survey Scientific Investigations Report (Black et al. 2023) and Data Release (Wright et al. 2023)
Washington State Department of Health, Water System Data Sentry Database
This analysis of vulnerability identifies areas in Puget Sound that are at higher risk of nitrate contamination in groundwater and describes how that risk has changed over time. For this study, groundwater vulnerability refers to the relative ease with which a contaminant applied at or near the land surface can migrate to the aquifer of interest for a given set of land-use practices.
This indicator builds off previous U.S. Geological Survey (USGS) vulnerability assessments to map the modeled probability of elevated nitrate concentrations in groundwater at 150-foot and 300-foot depths over four time periods: 2000-04, 2005-09, 2010-14, and 2015-19. Nitrate concentrations are considered elevated when they exceed 2 mg/L. This threshold indicates human activities are the source of the contaminant (Nolan et al., 1998).
The groundwater vulnerability assessment was developed in several steps for this indicator:
- USGS scientists compiled available nitrate concentration and depth data for Group A wells from the Washington Department of Health Sentry database (Figure 1).
- Nitrate concentrations in groundwater data were overlaid with land cover (NLCD) and other anthropogenic and hydrogeologic data using a GIS to produce a dataset in which each Group A well with a measured nitrate concentration was attributed with values of these variables within four time periods: 2000-04, 2005-09, 2010-14, and 2015-19. These data were then downloaded to a statistical software package for analysis.
- A range of empirical models (logistic regression) designed to predict the probability of elevated nitrate concentrations (those exceeding a threshold of 2 mg/L) in groundwater were developed based on the available nitrate concentrations in Group A wells, and the well depth, land cover, anthropogenic, and hydrogeologic variables for each of the four time periods.
- The empirical models that best estimated the probability of elevated nitrate concentrations in groundwater were identified and validated.
- Those empirical models were entered into a GIS, and probability maps for the entire Puget Sound Basin were constructed.
The predicted probabilities of exceeding the nitrate concentration threshold of 2 mg/L were graphically and statistically evaluated to identify potential indicators of change in vulnerability.
The resulting probability maps do not represent actual nitrate contamination of groundwater, but rather they show the probability that nitrate concentrations exceed 2 mg/L. The maps and predicted probability analyses in this report are intended for regional scale use and have limitations for use at the field-scale due to the coarse resolution of some explanatory variables.
For more information about the methods used for this indicator, please see Black et al. 2023.
Figure 1. Map of Washington Department of Health Group A well locations used to create the predictive models to determine the probability of elevated nitrates in groundwater at 150- and 300-foot depths. - Critical Definitions
Groundwater vulnerability refers to the relative ease with which a contaminant applied at or near the land surface can migrate to the aquifer of interest for a given set of land-use practices.
Logistic regression was selected for this analysis because it quantifies the relation between a variable of interest (the response variable, nitrate concentration in this case) and one or more variables that affect the variable of interest (explanatory variables). This is conceptually like multiple linear regression. However, the response variable in logistic regression is transformed into a binary response (yes or no) variable. This makes logistic regression an excellent tool for modeling aquifer vulnerability to nitrate contamination because logistic regression can quantify the probability that nitrate concentrations will exceed a specified concentration.
Based on modeling results, more than 75% of Puget Sound groundwater has less than a 10% probability of having nitrate levels equal to or greater than 2 mg/L (see the Vital Sign Indicator Chart above and Figure 2 below). Well depth, soil properties and the percent agricultural and urban land cover surrounding a well were the most significant explanatory variables of vulnerability for all four time periods. As well depth increases, the probability of elevated nitrate concentrations decreases. On the other hand, as agricultural or urban land cover increases around a well, the probability of elevated nitrate concentrations also increases. The consistent significance of these factors suggest that they are reliable predictors of groundwater vulnerability over time.
| Time Period | 150-foot Depth | 300-foot Depth |
|---|---|---|
| 2000-2004 | ||
| 2005-2009 | ||
| 2010-2014 | ||
| 2015-2019 |
Change Over Time
Figure 3 presents a series of maps to examine how the probability of elevated nitrate concentrations at specific locations changed over time. The change maps for 150-foot-deep wells show inconsistent trends over time. For example, probabilities of elevated nitrate concentrations mostly increased between 2000-04 and 2005-09 while probabilities mostly decreased between 2005-09 and 2010-14. Changes between 2010-14 and 2015-19 included a mix of increased and decreased probabilities.
In contrast, at the 300-foot depths, probabilities of elevated nitrates generally increased between all four time periods (Figure 3). The difference in responses between the 150- and 300-foot wells suggest that conditions in shallower wells are more dynamic than in deeper wells. Deeper wells are known to respond to land surface changes more slowly and therefore, may not be as representative of current anthropogenic changes. This is due to physical processes that delay groundwater recharge at deeper depths and chemical processes that alter the amount of nitrates reaching deeper depths.
We used statistical tests to examine trends in groundwater vulnerability to elevated nitrates over time in more detail, as described in Black et al. (2023) and shown in the Vital Sign Indicator Chart above. Between 2000 and 2019, a majority of the modeled Puget Sound groundwater had a very low probability (less than 25%) of elevated nitrate, but had a slight increase in these low probabilities over time. For groundwater with a probability of elevated nitrate between 25 and 70%, a decline in nitrate vulnerability occurred between 2000 and 2019. Groundwater with a high probability of elevated nitrates (greater than 70%) saw a significant increase between 2000 and 2019, but only accounted for approximately 1% of the Puget Sound groundwater.
| Time Period | 150-foot Depth | 300-foot Depth |
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2000-2004 to 2005-2009 |
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2005-2009 to 2010-2014 |
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2010-2014 to 2015-2019 |
Limitations
The maps and statistical analysis presented in this study provide a valuable and informative evaluation of the vulnerability of groundwater in the Puget Sound Basin to elevated nitrate concentrations. However, it is important to remember that the probability maps do not represent actual nitrate contamination of groundwater. Rather they present the probability that nitrate concentrations exceed 2 mg/L. The maps and predicted probability analyses reported for this indicator are intended for regional scale use and have limitations for use at the field-scale given the scale of the explanatory variables.
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Potential causes for the increased probability of detecting elevated nitrate in groundwater based on modeling results suggest increased urbanization and agricultural land cover, increased porosity of the soil and shallower depths. Lower probabilities of detecting elevated nitrates can be found at deeper groundwater depths and in areas with low urbanization and agricultural land cover.
Black, R.W., Wright, E.E., Bright, V.A.L., and Headman, A.O., 2023, Prediction of the probability of elevated nitrate concentrations at groundwater depths used for drinking-water supply in the Puget Sound basin, Washington, 2004–19: U.S. Geological Survey Scientific Investigations Report 2023–5117, 40 p., https://doi.org/10.3133/sir20235117.
Nolan, B.T., B.C. Ruddy, K.J. Hitt, and D.R. Helsel, 1998, A National look at nitrate contamination in ground water: Water Conditioning and Purification, v. 39, no. 12, p. 76–79.
Wright, E.E., Bright, V.L., Black, R.W., and Headman, A.O., 2023, Index of vulnerability for elevated nitrates in groundwater in the Puget Sound Basin, Washington, 2000–2019: U.S. Geological Survey data release, https://doi.org/10.5066/P9TOWGYM
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