ATRAZINE IN A STREAM-AQUIFER SYSTEM - ESTIMATION OF AQUIFER PROPERTIES FROM ATRAZINE CONCENTRATION PROFILES

Lincoln's municipal wellfield consists of 44 wells developed in an all uvial aquifer adjacent to the Platte River near Ashland, Nebraska. Ind uced recharge from the river is the primary source of water for the we llfield. Water samples were collected on a periodic basis from the Pla tte River and two transects of monitoring wells. These samples were an alyzed for the herbicide atrazine, which was used as a tracer of induc ed recharge in this stream-aquifer system. Atrazine concentrations in the river and aquifer were much less than 1.0 ppb during late fall and winter, but increased to as high as 18.9 ppb during spring and summer , associated with runoff from upgradient agricultural lands. There was approximately a 21-day lag time from the first detection of increasin g atrazine concentration in the river to the first detection in monito ring wells immediately adjacent to the river. This lag time was relati vely constant throughout the year, and from one year to the next, even with major fluctuations of river stage and wellfield production. This consistency of lag time indicated that the travel times from the rive r to the first set of monitoring wells immediately adjacent to the riv er were fairly constant. Paths of preferential flow were identified in the aquifer at a depth of 25 to 35 feet below land surface. This aqui fer zone appeared to play a significant role in movement of water from beneath the river into the wellfield. Aquifer dispersivity was calcul ated using a method described by Hoehn and Santschi (1987). Macrodispe rsivity (A(L)) was shown to increase linearly over the scale of the we llfield. Calculated values of A(L) were within limits of other reporte d values for this type of aquifer material and agreed well with values reported by Hoehn and Santschi (1987). These findings will be extreme ly beneficial for planning and management of the municipal wellfield.