Risk‐Based Selection of Monitoring Wells for Assessing Agricultural Chemical Contamination of Ground Water
Authors: Scheibe Timothy D. , Lettenmaier Dennis P. 
Published: February 2007

Monitoring of water supply wells has long been a responsibility of public health agencies. With the exception of large municipal water supply systems, most monitoring has been bacteriological, and sampling priority and frequency have been governed by the size of the water supply system. Current concerns about the health effects of agricultural chemicals has created a dilemma for public health agencies, because sampling costs for trace organic chemicals are much higher than for bacteriological parameters. A three‐level hierarchical strategy for jointly assessing contamination risk of water supply wells by agricultural chemicals, and prioritizing wells for sampling, is described. At the first level, a determination is made as to data needs and availability, monitoring objectives are defined, and the scope of the monitoring program is established. At the second level, risk or contamination susceptibility factors are identified, and algorithms for quantifying contamination risk as a function of the susceptibility factors are developed. At the third level, the algorithms are calibrated using pilot data, and an optimization scheme for prioritizing additional wells for sampling is developed based on either minimization of the aggregate health risk and risk uncertainty, or on information gained through sampling. The monitoring of Whatcom County, Washington, domestic wells for ethylene dibromide (EDB), a highly toxic soil fumigant whose registration was suspended in 1983 by the U.S. Environmental Protection Agency, is used as a case study for testing of Levels Two and Three of the hierarchy. In the case study, the initial data consisted of EDB concentrations and ancillary data, such as well depth and pumping rate, for 24 wells already sampled. Susceptibility factors were identified, and a Level Three algorithm was used to prioritize the next 10 wells from among 54 additional candidate wells for a second stage of sampling. The sensitivity of the monitoring well selection, and risk‐benefit tradeoff, are evaluated for different monitoring budgets, risk susceptibility parameters, and risk‐benefit tradeoff criteria.

The design of sampling programs for detection of agricultural chemicals in ground water is a difficult task. A review of ground water pesticide sampling programs that have been undertaken to date indicate that a lack of direction has existed in many past studies. This lack of focus generally has resulted from failure to clearly define sampling program objectives and to relate data collection priorities to those objectives. Even in California, where a very large sampling program for DBCP was undertaken (more than 8000 wells sampled), Holden (1986) reported that due to the lack of specific study objectives, the data obtained were limited in the scope of their applicability. In this paper, a methodology has been described that provides a framework for clear definition of objectives and data needs, enabling efficient design of sampling programs. The framework consists of a three‐level hierarchy. Once study objectives and data needs have been defined at the top level of the hierarchy, specific analytical methods are used (in the second level) to link well contamination probability to certain susceptibility factors and (in the third level) to select wells for sampling in a multistage sampling process.

The methodology was applied to a specific problem, the contamination of drinking water in Whatcom County, Washington, by the pesticide EDB. The application study demonstrated the use of available data to estimate the probability of contamination for each well in the study area, and the use of such estimates to guide future sampling as part of a multistage, interactive sampling program. The proposed approach is generally applicable to the problem of ground water contamination by agricultural chemicals. Many pesticides and herbicides are currently of concern because of their potential presence in drinking water. Sampling programs currently being undertaken to study these problems could clearly benefit from structuring of the monitoring design decision process as proposed and demonstrated.