RELATIONSHIP BETWEEN MINERALOGY AND FUNGICIDE/DEGRADATE CONCENTRATIONS IN SATURATED ZONE SEDIMENTS, NEW JERSEY COASTAL PLAIN—IMPLICATIONS FOR LONG-TERM EFFECTS ON GROUNDWATER QUALITY

The fate and transport of fungicides and their transformation products (degradates) from sites of application (cropland) into underlying aquifers requires evaluation of the mineralogy of sediments in the unsaturated and saturated zones because solid phases (clays, oxides/hydroxides, or organic matter) may adsorb these compounds. Contaminant sorption can sequester compounds temporarily or long term, which can then retard or inhibit transport to aquifers. Chlorothalonil, a widely used fungicide in the US, is primarily applied to peanuts and potatoes. Seven sites across an 8 km² agricultural research station in the New Jersey Coastal Plain, where chlorothalonil was applied, were cored and sampled to evaluate the possible role of mineralogy in the fate and transport of chlorothalonil or its degradates in the underlying Miocene Bridgeton Formation. The research station was selected for study because detailed records of chlorothalonil applications have been maintained for fields in which various crops were grown.

Detrital grains in the Bridgeton—dominantly quartz (>80 wt%) and feldspar (≤6 wt%)—are coated by a thin layer (≤50 μm across) of a complex mixture of minerals. Dry sieving reveals that minerals in the coatings comprise ≤ 6.23 wt% of the sediment. Quantitative X-ray diffraction (XRD) of the coatings indicates that they are composed of kaolinite ± halloysite (8-52 wt%), goethite (0-47 wt%), gibbsite (0-23 wt%), quartz (18-71 wt%), potassium feldspar (4-25 wt%) and illite (0-10 wt%). Scanning electron microscope examination confirms the mineralogy of coatings determined by XRD. Total organic carbon content of the sediments is very low (0.04-0.12 %). Chlorothanil was detected in only one sediment sample (5.49 ng/g), whereas its degradates were detected much more frequently and at higher concentrations (≤ 18.2 ng/g). Variably-charged minerals may be an important sink for chlorothalonil degradates in the absence of organic carbon. For example, one degradate (1-amide, 4-hydroxychlorothalonil) was associated primarily with samples that have >41% kaolin group minerals in sediment coatings, whereas another degradate (1,3-diamide chlorothalonil) was in samples with >3% goethite in the coatings. These results may point to degradate interaction with some types of polar minerals that exist in the coatings.