National Assessment of Long-Term Groundwater Response to Pesticide Regulation

Abstract

Quantitative assessments of long-term, national-scale responses of groundwater quality to pesticide applications are essential to evaluate the effectiveness of pesticide regulations. Retardation time in the unsaturated zone (R_u) was estimated for selected herbicides (atrazine, simazine, and bentazon) and degradation products (desethylatrazine (DEA), desisopropylatrazine (DIA), desethyldesisopropylatrazine (DEIA), and BAM) using a multidecadal time series of groundwater solute chemistry (∼30 years) and herbicide sales (∼60 years). The sampling year was converted to recharge year using groundwater age. Then, R_u was estimated using a cross-correlation analysis of the sales and the frequencies of detection and exceedance of the drinking water standard (0.1 μg/L) of each selected compound. The results showed no retardation of the highly polar, thus mobile, parent compounds (i.e., bentazon), while R_u of the moderately polar compounds (i.e., simazine) was about a decade, and their degradation products showed even longer R_u. The temporal trends of the degradation products did not mirror those of the sale data, which were attributed to the various sale periods of the parent compounds, sorption of the parent compounds, and complex degradation pathways. The longer R_u in clayey/organic sediments than in sandy sediments further confirmed the role of soil-specific retardation as an important factor to consider in groundwater protection.

Keywords: groundwater; lag time; national assessment; pesticides; retardation time; transport time.

Figure 1

Conceptual model of the retardation time analysis of this study.

Figure 2

Flow chart of the data processing protocol.

Figure 3

Location of the sampling points (well screens) of the final data sets as distributed among the 10 Danish georegions, which is indicated with roman numerals. The surface geology/sediment type at a 1 m depth is shown in the background.

Figure 4

Time series of frequencies of detection and exceedance of 0.1 μg/L of atrazine, simazine, DEA, DIA, DEIA, BAM, and bentazon across all wells (black) and divided into sandy sediments (red) and clayey + organic sediments (blue). Sale data (kg/year) of parent compounds are shown in gray bars.

Figure 5

Histogram of groundwater age under sandy sediments (a) and clayey/organic sediments (b) by redox conditions of groundwater (left y-axis). The mean ages and standard deviations are shown in each panel. The number of groundwater age measurements is shown in parenthesis as well as a solid black line in each panel (right y-axis). The results of the 10 well screens underlain by other types of sediments are not shown here. DO: dissolved oxygen. The depth distributions of this study’s well screens and the drinking water abstraction wells in Denmark are shown in (c) as depth to the center of the screen as meter below the land surface (m bls).