Nitrogen (N) fertilizer applications have resulted in widespread groundwater nitrate-N (NO 3 -N) contamination in the U.S. Corn Belt. Goodwater Creek Experimental Watershed (GCEW) is an agricultural watershed in the claypan soil region of northeastern Missouri with a network of 96 wells at depths of 2.7-15.7 m. The objectives of this study were to (1) inspect the spatial and temporal variations of NO 3 -N concentrations in GCEW's groundwater, particularly with well depth at scales ranging from individual well, well nest, and field to the entire watershed during the period 1991 to 2004; (2) understand the processes controlling the variability of NO 3 -N concentrations in groundwater at various scales within GCEW; and (3) compare groundwater NO 3 -N concentrations in GCEW to other agricultural watersheds in the U.S. Nitrate-N concentrations were determined in more than 2000 samples collected from 1991 to 2004. Despite the low hydraulic conductivity of the claypan soils, considerable NO 3 -N contamination of the glacial till aquifer occurred, with 38% of the wells exceeding 10 mg L À1 . Groundwater recharge by preferential pathways through the claypan appeared to be the primary mechanism for NO 3 -N movement to the aquifer. Changes in concentration with depth steadily increased to 8.5-10 m and then decreased with further depth. This pattern was consistent with decreased hydraulic conductivity in the Paleosol layer at 8.5-10 m, denitrification below this layer, and mixing of recent contaminated water with older uncontaminated water in the lowest strata. Only 19-23% of sampled wells exceeded 10 mg L À1 in nonclaypan agricultural watersheds over the continental U.S., suggesting that groundwater in GCEW was more susceptible to NO 3 -N contamination than nonclaypan watersheds. These results demonstrated that preferential flow through the soil and hydraulic conductivity of the subsurface strata controlled NO 3 -N transport in this claypan watershed.

a b s t r a c tThe study investigates how water chemistry evolves as ephemeral stream runoff is formed through the interaction of sediments and precipitation in the Amargosa Desert region and by analogy other desert regions. In this study, thirty lysimeters were installed in the major arroyos in the Amargosa Desert to capture runoff water. The sampling process included sediment, precipitation, and runoff water chemistry. Innovative and low cost methods were used to measure the chemical composition of the resulting runoff and examined some of the important processes affecting the runoff chemistry. Results of the analytical and statistical analyses indicate that runoff salinity is low as a result of net salt accumulation in sediments. Chemical behavior between precipitation and runoff is classified as leached (TDS, alkalinity, Ca, Mg, K, Na, Ba, Cs, Li, Sr, Fe, Ni), nutrient (Br, As, SO 2À 4 , PO 3À 4 , NO À 3 , Rb, B, Cu, Zn, V), scavenged (U, F), and conservative (Al, Mo, Mn). Bromide behaves as a nutrient meaning the chloride/bromide ratio, a common tracer of groundwater sources, is not conservative. Runoff chloride, sulfate, and sodium are predominantly associated with concentrations of the same ions in sediment. Trace elements are more closely associated with precipitation chemistry.

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