Testing Whether Rock Dams Cause Rapid Recharge

Abstract

Groundwater resources are under stress due to increased pumping observed in an increasingly arid American southwest. When more water is pumped out than is recharged, this results in over pumping which causes drying wells, land fissures, and land subsidence. Therefore, it is important to look for ways in which to conserve and replenish our aquifers. One possible mitigation strategy is the use of rock dams, which slow the flow of water and promote infiltration beneath ephemeral streams. Studies have shown that rock dams can improve riparian health, however, it is unclear whether these structures can cause recharge of groundwater over short timescales (months). The sooner that aquifers can be recharged, the sooner aquifers can begin to recover, drawdown can be mitigated, and land subsidence and fissures can be avoided. Our hypothesis was that rock dams could cause rapid recharge (recharge within 3-months) and that mounding of the water table could be an indication that rapid recharge was occuring. To test this hypothesis, water samples were collected during the 2021 summer monsoon season and analyzed for major anions and cations, stable water isotopes (δ18O and δD), and tritium (3H). The water chemistry and isotope data did not show any evidence of rapid recharge. Using HYDRUS-1D, different soil textures and streamflow durations were modeled to determine the theoretical conditions that would allow for rapid recharge. Based on the modeling results, we found that only soils that contained little to no clay would allow for recharge on the scale of months. However, our models used several simplifying assumptions and considering real-world complexities, we would not expect to see recharge on the scale of months, but rather years. To determine true recharge rates of rock dams, future monitoring systems could do tritium testing on the scale of years and/or install additional sensors (pressure or water content) in the subsurface to track infiltration rates on a longer timescale.