Apparent Concentrations of Chloride, Sulfate and Nitrate in the Edward's Aquifer Recharge Zone

For our study we used public data from the Texas Water Development Board for sulfate, nitrate, and chloride compound concentrations. While these compounds are not immediate threats to the public’s health and well-being we chose to focus on them at levels which may yield cause for concern. Concentrations are represented in the density unit milligram per liter.

For nitrate we chose to focus our study on levels higher than 10 mg/L. This is the standard set by the Environmental Protection Agency. Concentrations of nitrate above this level can be hazardous to human health and are especially dangerous for pregnant women and infants. Previous studies have shown that there is a correlation between nitrate levels above 10 mg/L and a condition known as methemoglobinemia, blue baby syndrome, where blood lacks the ability to carry enough oxygen to cells throughout the body and cause the baby’s skin and veins to appear blue in color.

While chloride is a compound that is typically considered a background trace element, it can also indicate contamination from human or animal waste, which also contain high amounts of chloride. The recommended maximum value for Chloride in drinking water set by the EPA is 250 mg/L. Concentrations above this level can cause water to have an unpleasant salty taste and can lead to the corrosion of distribution systems for drinking water. Chloride is also associated with sodium, which may negatively affect those on sodium-restricted diets.

Sulfate is another element that is commonly found in drinking water, however concentrations exceeding 250 mg/L are known to have a laxative effect on humans and animals and can lead to dehydration and diarrhea. Elderly and children are most susceptible to these effects, however healthy humans are also known to be able to adapt to some elevated sulfate levels after the initial adverse side effects. Sulfate, like Chloride can also cause corrosion of pipes.

Table 1. Concentration Table

This table details the concentrations of each compound that we chose to focus on for our project and our interpretation. NOTE: Due to limited data access, the project used public data for sulfate, nitrate, and chloride compound concentrations. Those compounds are not as health concerning as Arsenic. For the purpose of the project, high compound concentrations were used to represents the same objective of the project. TWDB public data was used for Sulfate, Nitrate, and Chloride concentrations values. Concentrations are represented in density unit, mg/L.

The Texas Water Development board defines pollution as the alteration or contamination of the physical, thermal, chemical, or biological quality of any water in the state, or the contamination of any water in the state, that renders the water harmful, detrimental, or injurious to humans, animal life, vegetation, property, or public health, safety, or welfare or that impairs the usefulness of the public enjoyment of the water for any lawful or reasonable purpose (Board, 2005).

The span of our project is defined by the monitored wells by the Texas Water Development Board within Bexar, Comal, Medina, and Uvalde counties. High concentrations for our selected compounds are labeled as contaminated because they are above normal background levels.

The data we obtained ranged from the 1950s to 2015. The primary objective is to observe the change of water quality over time for our selected element concentrations. In order to accomplish this and easily identify trends in our data, we divided our data into 20 year intervals. The three time intervals are from the year 1955 to 1975, 1975 to 1993, and 1993 to 2015. Possible sources that might have led to the contamination, such as landfills, cattle and livestock feedlots, water treatment plants, and golf courses are depicted and potentially vulnerable areas surrounding them are delineated by buffers. The data for well locations were obtained from the TWDB website and converted to an Excel File. Worksheets were divided in Excel by years. The x and y coordinates for decimal longitude and latitude were imported into ArcMap.