Groundwater contaminants are on the increase in many locations across the country, including Missouri, according to a 2012 report from the U.S. Geological Survey (USGS).
The USGS National Water-Quality Assessment Program monitors water quality in well networks across the country. The report looked at chloride, dissolved solids, and nitrate concentrations in water samples from 1,235 wells in 56 well networks. Concentrations of these three compounds are indicators of overall water quality and the potability of drinking water. Because they were interested in changes over time, the report compared concentrations from the decade before the Millennium (1988-2000 actually) and the decade after the Millennium (2000-2010).
Chloride is a constituent of salt. It is of concern because too much chloride in drinking water gives it a salty taste, and because it is a health risk. Dissolved solids are of concern because they also give drinking water a bad taste, they make the water “hard,” and they cause mineral deposits and staining. Nitrates are of concern because elevated levels in drinking water cause health problems.
All three compounds can get into groundwater from natural sources. However, contamination can also be related to human sources. For instance, chloride can seep into groundwater from deicing compounds applied to roads or from salts in septic systems. Primary human sources of dissolved solids include agricultural runoff, leaching of soil contaminates, and industrial/sewage plants. Nitrates are the most frequent human-caused groundwater contaminant, deriving from fertilizers used in farming and landscaping.
Whatever their source, to protect our drinking water, it is important to keep our groundwater as free of contaminants as possible.
Taking all three compounds together, 66% of the well networks tested showed a statistically significant increase in at least 1 of the compounds. Each of the compounds is discussed below.
Figure 1 shows the results for chloride. Each well network is located with a label: “lusag,” “lusurb,” or “sus,” depending on the type of well network (the different types of well networks are unimportant for the purposes of this blog post). Well networks with a green arrow showed a decrease in chloride concentration. Networks with a red arrow showed an increase. Networks with a black dot showed no statistically significant change. The size of the arrow represents the size of the change. Colored regions represent the type of aquifer from which the groundwater came.
(For larger view, click on figure.)
The lone tested well network in Missouri showed an increase in chloride concentrations, but not large enough to be statistically significant. Nationally, 24 out of 56 well networks had a statistically significant increase. Only two had statistically significant decreases.
Figure 2 shows the results for dissolved solids. The well networks are labeled as in Figure 1, except in this map, yellow highlighting around the well network indicates networks where the level of dissolved solids was greater than 500 mg/L in the second sampling. Five hundred mg/L is the Secondary Maximum Concentration Level (SMCL) for drinking water, meaning that drinking water from these locations requires treatment to reduce the level of dissolved solids.
The network tested in Missouri experienced a statistically significant 24 mg/L increase in dissolved solids. The report does not give the absolute value of dissolved solids, but the absence of yellow highlighting indicates that the concentration was not greater than 500 mg/L. Nationwide, 22 out of 54 well networks tested experienced a statistically significant increase in dissolved solids. Only 1 had a statistically significant decrease.
Figure 3 shows the results for nitrates. The well networks are labeled as in Figure 2, with yellow highlighting around well networks indicating networks where, in the second sampling, the concentration of nitrate exceeded 10 mg/L, the Maximum Concentration Level for drinking water.
The network tested in Missouri showed a small increase that was not statistically significant. Nationally, 13 out of 56 well networks tested experienced a statistically significant increase, while 5 had a statistically significant decrease.
The data in this report are not meant to identify specific sources of groundwater contamination, but rather to indicate regional trends over time. The report is clear: few networks experienced decreases in the contaminants, and 2 out of 3 experienced a statistically significant increase in at least one of them.
Lindsay, Bruce, and Michael Rupert. 2012. Methods for Evaluating Temporal Groundwater Quality Data and Results of Decadal-Scale Changes in Chloride, Dissolved Solids, and Nitrate Concentrations in Groundwater in the United States, 1988-2010. United States Geological Survey Scientific Investigations Report 2012-5049, http://pubs.usgs.gov/sir/2012/5049.