Quantifying Groundwater Quality at a Regional Scale: Establishing a Foundation for Economic and Health Assessments

Abstract:

What is the value of clean groundwater? Might one aquifer be considered more valuable than another? To help address these, and similar, questions, we propose that aquifers be assessed by two metrics: 1) the contaminated area of an aquifer, defined by high concentrations (km2 or proportion); and 2) equivalent-population potentially impacted by that contamination (number of people or proportion). Concentrations are considered high if they are above a human health benchmark. The two metrics provide a quantitative basis for assessment at the aquifer scale, rather than the well scale.

This approach has been applied to groundwater used for public supply in California (Belitz and others, 2015). The assessment distinguishes between population (34 million, 2000 census) and equivalent-population (11 million) because public drinking water supplies can be a mix of surface water and groundwater. The assessment was conducted in 87 study areas which account for nearly 100% of the groundwater used for public supply.

The area-metric, when expressed as a proportion, is useful for identifying where a particular contaminant or class of contaminants might be a cause for concern. In CA, there are 38 study where the area-metric ≥ 25% for one or more contaminants; in 7 of these, the area-metric ≥ 50%. Naturally-occurring trace elements, such as arsenic and uranium, are the most prevalent contaminant class in 72 study areas. Nitrate is most prevalent at high concentrations in 11 study areas, and organic compounds in 4. By the area-metric, 23% of the groundwater used for public supply in CA has high concentrations of one or more contaminants (20,000 of 89,000 km2 assessed).

The population-metric, when expressed as a number of people, identifies the potential impact of groundwater contamination. There are 33 CA study areas where the population-metric exceeds 10,000 people (equivalent population multiplied by detection frequency of wells with high concentrations). The population-metric exceeds 50,000 people in 10 study areas. On a statewide basis, the population metric is 2 million people (18% of 11 million equivalent-people).

The proposed assessment approach is independent of scale, allows for consistent comparison across regions, and provides a foundation for subsequent economic or health assessments.