A Subregional‐Scale Method to Assess Aquifer Vulnerability to Pesticides

Abstract

A method to predict aquifer vulnerability to pesticide contamination at the subregional scale was developed. The assessment method was designed to incorporate relevant hydrologic and pesticide-transport information and to use generally available data. The method assumes steady-state advection of pesticides in the vadose zone, including sorption and biological decay. The solution is presented as a vulnerability index (VI) that increases as the aquifer vulnerability increases. The hydrologic input data for the VI model are obtained from the soil survey geographic database. Pesticides were grouped into three leachability classes using a leachability ratio (half-life divided by organic carbon partition coefficient). Pesticide transformation is assumed to occur in the surface layer. The influence of vertical transport in the remainder of the vadose zone has been incorporated by applying a multiplying factor to the VI that varies with depth to ground water. Hydraulic conductivity is used as a surrogate for soil-water velocity for practical purposes. The performance of the VI model is evaluated using ground water data from Weld County, Colorado. The model is demonstrated to be successful at predicting relative vulnerability, defined as the magnitude of pesticide concentration and percent of wells in a unit that exhibit a pesticide detection. Areas of low, medium, and high vulnerability are assigned. Results indicate that the vulnerability classifications are most dependent on the leachability ratio, hydraulic conductivity, and organic carbon content.

The vulnerability of groundwater to pesticide contamination estimated directly from observations of presence or absence in wells

“…Shukla et al (2000) when using a method based on the attenuation factor of Rao et al (1985) showed that although there was general agreement between vulnerability prediction and observed groundwater contamination, there were insufficient data to provide a sufficient test, there was general agreement. Schlosser et al (2002) developed a vulnerability system based on a vulnerability index of pesticides and showed a good agreement with groundwater observations, but did not provide any statistical test of their validation. Indeed, none of these attempts at validation provide a statistical test of the indices.…”

Pesticide Leaching Potential in a Shallow Unconfined Aquifer

“…Kookana and Aylmore (1994) modified the model of Jury et al (1987) for decreasing organic matter contents in soil with respect to depth. Recently, Schlosser et al (2002) presented a modified method of Meeks and Dean (1990) for the assessment of aquifer vulnerability to pesticide leaching and introduced the term "vulnerability index" (VI). These methods are based on the one-dimensional advective-dispersive transport equation for a non-conservative chemical that follows first order decay and linear adsorption in soils.…”

“…Other qualitative assessment schemes include those of Teso et al (1988), Le Seur et al (1987), and Breeuwsma and Duijvenbooden (1987). Methods that offer a more quantitative assessment of groundwater vulnerability include those of Dean et al (1984), Rao et al (1985), Meeks and Dean (1990), Kookana and Aylmore (1994), Di et al (1995) and Schlosser et al (2002). Anwar and Bari (2004) prioritized areas for groundwater monitoring based on leaching of three pesticides.…”

Statistical screening for descriptive parameters for pesticide occurrence in a shallow groundwater catchment

“…In terms of environmental pesticide risk indicator schemes however, this parameter as well as other site parameters such as depth to groundwater table and distance from well to point of application are less commonly adopted. They are, however common vulnerability measures in terms of overall groundwater vulnerability assessment schemes (Aller et al, 1987;Schlosser et al, 2002;Worrall and Kolpin, 2004).…”

“…The hydraulic conductivity of the treated sediments, the distance to well and to the groundwater surface from the point of application may likely constitute suitable descriptive variables in terms of relative risk assessment of groundwater contamination with regard to comparisons between catchments of varying physical and land-use properties, or between single pesticide applications (e.g. Barbash and Resek, 1996;Schlosser et al, 2002;Tiktak et al, 2004). However, this is likely not the case when attempting relative environmental risk assessment for the use of specific pesticides within relatively small areas dominated by rotational cropping practices such as the catchment studied.…”