ARS | Publication request: MULTIPLE TRACER ANALYSIS OF WATER RECHARGE IN DEEP LOESS

Submitted to: Ground Water
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: January 12, 1995
Publication Date: N/A

Interpretive Summary: There is increasing concern over the potential for agricultural chemicals to leach into groundwater supplies. It is generally assumed that in dryland areas such as eastern Washington the occurrence of leaching chemicals would be low since we have low rainfall. However, we have shown that chemicals may leach through large cracks and pores in soil and may be transported quickly even in low rainfall conditions. In addition it has been shown that upwards of 25% of the precipitation in this area is recharging groundwater. To further our knowledge of this system we explored tracer analysis to indicate how water moves in these deep soils and thus how chemicals could move toward groundwater. We found that due to complex flow patters with respect to slope position chemicals may move at substantially different rates and may be affected by biological processes. We are continuing our study of water and chemical flow using a transect of monitoring wells to indicate rate and direction of chemical movement.

Technical Abstract: Vertical profiles of chloride, tritium and nitrate porewater concentrations were determined to 8 m depth across a loess hillslope. Mean recharge fluxes, estimated from chloride mass balance, are 5-10 times larger at the mid- and toe-slope positions than at the top slope; the magnitudes of the values compare favorably with results from other methods. The tritium and nitrate profiles exhibit multiple peaks which indicate that piston flow is not the sole flow process in this system. The importance of dispersive processes is confirmed by great dilution of tritium concentrations, relative to expected concentrations assuming piston flow. In this setting, chloride is useful as a recharge estimator while tritium and nitrate serve as tracers of landscape-level water movement.