UNTANGLING THE WEB OF GEOPHYSICAL MEASUREMENTS OF WATER SALINITY/SATURATION NEAR THE RIO GRANDE, WEST TEXAS

Electrical methods are commonly used to track the flow of water and contaminants in the shallow subsurface. We have repeated geophysical surveys at five sites, representing a range of depositional environments along the Rio Grande, with borings collected at these sites for laboratory analysis of grain-size and salinity. We have found that resistivity measurements vary seasonally, but are not repeatable even at yearly cycles. The seasonal influences include river level, high evaporation rate, and temperature changes. To complicate matters, the changes can occur over a period of days, with complex controls from salinity, saturation, and grain size. Resistivity is not a unique/reliable indicator of water level, salinity, or grain size. The one unique geophysical measurement is the control of magnetic intensity by the presence of magnetite in coarse-grained channel deposits. The larger resistivity changes, directly related to river water level changes, generally occurred in the coarse-grained, thickly-bedded channel deposits. Highly variable, shallow resistivity variations commonly occurred in more thinly bedded crevasse splay deposits: these show the effect of remote irrigation water moving through splay channels, and salt precipitation and re-solution in finer-grained deposits where the capillary fringe is near ground level. Fine-grained flood-plain deposits generally show a stable low resistivity over both short-term and yearly seasonal changes.