Abstract
A field study was conducted to determine the applicability of multivariate geostatistical methods to the problem of estimating pesticide concentrations in groundwater from measured concentrations of nitrate and pesticide, when pesticide is undersampled. Measured concentrations of nitrate-N and the herbicide Dimethyl tetrachloroterepthalate (DCPA) were obtained for 42 wells in a shallow unconfined alluvial and basin-fill aquifer in a 165 km2 agricultural area in eastern Oregon. The correlation coefficient between log(nitrate-N) and log(DCPA) was 0.74. Isotropic, spherical models were fitted to experimental direct- and cross-semivariograms with correlation ranges and sliding neighborhoods of 4 km. The relative gain for estimates obtained by cokriging ranged from 14 to 34%. Additional sample locations were selected for nitrate and DCPA using the fictitious point method. A simple economic analysis demonstrated that additional nitrate samples would be more beneficial in reducing estimation variances than additional DCPA samples, unless the costs of analysis were identical.
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