New Mexico Geological Society Annual Spring Meeting — Abstracts

 In the southwestern U.S., input of deeply-circulated groundwater has been shown to contribute to surface water salinization, degrading water quality. Fault structures can provide conduits for up- flow of these fluids.

Recent studies of the Jemez River, NM, indicate that where the river crosses faults, salinity and deeply derived volatiles increase, (e.g., prominent spring inputs at Soda Dam and concomitant production of travertine). In this tributary system to the Rio Grande, groundwater inputs profoundly affect surface water chemistry, raising concentration levels of several elements above EPA standards, particularly during low-flow conditions.

Rio Salado near San Ysidro provides another location to examine this phenomenon. Where the Rio Salado crosses the Nacimiento Fault, several springs discharge along the fault to the south and north of the river. Because the springs flow along the fault structure within and at a distance from the river, the effects of groundwater flow can be examined in greater detail. We report here the initial results of examining the hydrochemistry of the Salado and Nacimiento fault springs.

Two strategies were used, continuous monitoring of key hydrochemical parameters (temperature, specific conductance and depth) in four springs along the structure (including both sides of the river) and water chemistry, collected over a multi-year period. Seasonal and diurnal variations are seen as well as systematic depth (discharge) changes. Monitoring data highlight connectivity of three springs along the fault, while the fourth shows potential anthropogenic influences.

Fluid connectivity between springs is highlighted most prominently by near-synchronous changes in water depth of each spring. Seasonal variations are seen at all locations with 3 of the 4 springs displaying diurnal changes. Specific conductance does not correlate with water temperature or depth at all locations. This, in part, could be due to bioaccumulation on the data logger, reducing the accuracy of the reading. At one location, specific conductance shows two cyclical patterns which vary on the time scale of hours and days. During the summer months, both cycles of variation are present, while only the shorter-term variations are noted during the rest of the year.

Major ion chemistry from water samples collected over a multi-year period shows similar hydrochemical facies for three springs west of the river while samples from the east and the Rio Salado show greater temporal variability. Stable isotopes of water (dD and d18O) and carbon, as well as gas chemistry are also reported. The geochemical data support previous geophysical experiments at the site showing circulation patterns of saline fluids in the fault zone as well as showing slight temperature increases in the Rio Salado where it crosses the fault zone. Overall these combined data indicate that groundwater movement along faults is an important process that influences water quality in surface and aquifer waters.