Natural‐Gradient Transport of Bromide, Atrazine, and Alachlor in an Organic Carbon‐Rich Aquifer

Abstract

In situ values of retardation factors and dispersivity were determined from a field tracer study using bromide, atrazine (2‐chloro‐4‐ethlamino‐6‐isopropylamineo‐ s ‐triazine), and alachlor (2‐chloro‐2′‐6′‐diethyl‐ N ‐(methoxymethyl)‐acetanilide). The tracers were injected at the water table in an unconfined aquifer to evaluate how these pesticides are attenuated within the groundwater flow system in a typical alluvial valley setting in the Midwest where corn ( Zea mays L.) and soybean [ Glycine max (L.) Merr.] are the dominant crops and atrazine and alachlor are the most commonly used pesticides. Water samples were collected for 49 d after tracer injection from 63 wells to establish the direction and magnitude of tracer movement. Following injection, no degradation products of atrazine were detected in any of the monitoring wells. Aquifer hydraulic properties were measured in the field prior to the test. Sorption on organic carbon (0.0009–0.0016 Mg/Mg) in the aquifer caused the transport of atrazine and alachlor to be retarded relative to bromide. At the scale of the field test, retardation factors of 6.1 and 7.1 were estimated for atrazine and alachlor, respectively, based on a one‐dimensional, nonlinear least squares inversion analysis. These retardation factors are two to three times larger than retardation factors measured in laboratory batch tests using the same aquifer materials, are larger than other values reported in the literature, and are due to the relatively large fraction of organic C found in the aquifer. Dispersivity was approximately 9 cm at a distance of 1 to 2 m from the injection well and approximately 140 cm at a distance of 5 m from the injection well.