Research - Category III: Water Quality

Nature of Flow and Gas Dynamics below Spreading Ponds
(Funded 2002-2003)

Principal Investigator: Jordan F. Clark
Department of Geology
UC Santa Barbara
(805) 893-7838
clark@magic.geol.ucsb.edu

Executive Summary:
Groundwater has been a primary source of potable and irrigation water in arid regions for centuries. During the last 50 years, the soaring demand for freshwater has placed unprecedented stresses upon many aquifers in California and elsewhere. One recent development in groundwater/surface water management aimed at augmenting water supplies is Aquifer Storage and Recovery (ASR) projects. This practice, which is also known as groundwater banking, consists of recharging recycled or surplus surface water into permeable aquifers and extracting this recharged water at some later date. As a result, ASR has become an important method for the combined management of surface and ground water. It has also become an important method to control salt water intrusion in coastal aquifers, raise water tables, and to maintain base flow in streams.

Fundamental issues concerning ASR are 1) water quality and 2) the potential impairment of the aquifer. This is especially true when recycled (or reclaimed) water is a component of the water supply for the ASR operation. As a result of these concerns, the California Department of Health Services has issued draft regulations for groundwater recharge reuse. In addition to specifying surface supply controls (i.e., treatments levels for waste water, dilution factors, etc.), these draft regulations consider aquifer processes that may change the quality of the recharging water. Most importantly, they recognize that in situ biogeochemical processes may act as an additional treatment step that could improve water quality. These biogeochemical processes are not well understood. In particular, there are conflicting data concerning whether the bulk of the flow beneath spreading ponds occurs primarily in saturated portions of the Vadose zone or if unsaturated flow dominates. The nature of this flow will determine the types of biogeochemical processes. It will also significantly affect the behavior of dissolved gases such as VOC, oxygen, and THMs. Furthermore, a detailed understanding of dissolved gas dynamics is critical when trying to design and interpret the behavior of gas tracers such as sulfur hexaflouride and noble gas isotopes. Ex0periments using these gas tracers have become acceptable methods for evaluating travel times from recharge locations to wells, important criteria used during the permitting processes of ASR operations in the State of California.

Here, I am proposing to conduct a series of dual gas tracer experiments below spreading ponds to investigate the nature of flow and dissolved gas dynamics through the Vadose zone. Because the transfer rates of gases from water into air are dependent on their diffusion coefficients, the change in the ratio of two gases that have different diffusion coefficients is proportional to the amount of gas transfer. Little gas loss will indicate that the flow is predominately saturated. The experiments will be performed in the El Rio Spreading Grounds, Ventura County, following periods of no recharge and periods of active recharge to assess if the nature of flow changes with antecedent (e.g. the amount of wetting) conditions. Results of the proposed work should impact water reuse regulations and interpretations of dissolved gas tracer experiments.