Empirical Modeling of Spatial Vulnerability Applied to a Norflurazon Retrospective Well Study in California

Abstract

One goal of mandated well monitoring in California, U.S.A, is to search for residues of active ingredients previously undetected in the state's groundwater. The realization that pesticide residues move into groundwater via a number of different pathways has lead us to develop an empirical approach to delineate vulnerable areas; major climatic and edaphic features of areas where pesticides residues have been detected in well water have been identified on a geographic basis. The objective of this study was to evaluate the use of our empirical model in a retrospective well sampling study for norflurazon, a pre-emergence herbicide with physical-chemical properties that indicated potential to move offsite with water. In our modeling approach, sections of land, which are 2.59 km 2 areas, were identified as having a greater potential for contamination based on soil and depth-to-groundwater data. Wells were sampled from a subset of these sections where use of norflurazon was historically the greatest. Norflurazon residue was detected in 8 of 43 wells sampled in Fresno County, CA., and in concentrations ranging from 0.07 to 0.69 µg L-1. This result was considered highly successful because residues had not been detected in 18 previous California groundwater studies for other active ingredients, some of which had been detected in other state and federal sampling programs. Location of sampling sites in these previous 18 California studies was based only on pesticide use data. The detections of norflurazon in this study indicated that, even though using an empirical modeling approach appeared to be unorthodox, it enabled us to effectively identify vulnerable areas.

Evaluating the use of digital terrain modelling for quantifying the spatial variability of 2,4-D sorption by soil within agricultural landscapes

“…Herbicide sorption has been shown to influence herbicide volatilization, degradation and leaching rates, and herbicide uptake by plants and soil fauna (Walker 1972;Baker et al 1996;Barriuso et al 1997). Since herbicide models increasingly incorporate spatial variability (Troiano et al, 1999;Wu and Workman 1999), it is essential to evaluate the distribution of herbicide sorption within landscapes. With increasing technology and capability of precision farming, understanding herbicide sorption by soil at the large-scale may lead to a reduction in herbicide movement off-site and the amount of herbicide needed (Khakural et al, 1994).…”

Modeling Herbicide Movement to Ground Water in Irrigated Sandy Soils of the San Joaquin Valley, California

“…Herbicide detection frequencies in ground water of Fresno and adjacent Tulare Counties are inversely correlated with DGW (Troiano et al, 1999). Figure 4 compares modeled and observed detection frequencies and 85th concentration percentiles for the two herbicides in the study area, where both demonstrate a like dependence on DGW (Figure 4).…”

“…Geostatistical methods used to derive the DGW data are summarized in Troiano et al (1999). The sampling distribution for D is based on the subset of kriging results for the study area.…”

“…Nonpoint source pesticide contamination of California ground water occurs under several conditions (Troiano et al, 1999). Coarse soil overlying shallow ground water is one condition where detections are relatively frequent.…”

Pesticide and Nitrate Trends in Domestic Wells where Pesticide Use Is Regulated in Fresno and Tulare Counties, California

“…Two pathways to groundwater were identified and geographically located based on soils data. Leaching of residues downward from surface applications was identified as the major pathway to groundwater in areas where coarse‐textured soils were predominant (Troiano et al, 1999). However, groundwater contamination was also shown to occur in areas where soils had low water infiltration rates: runoff water generated from either rainfall or irrigation and containing dissolved pesticide residue was identified as flowing into sensitive sites located near a field, such as to a drywell or pond.…”