UC Santa Barbara

Managed aquifer recharge (MAR) is an increasingly popular strategy for maintaining a sustainable water supply. Reclaimed water (sometimes referred to as recycled water) can be used as a supply of recharge water, providing environmental benefits, while natural processes improve the quality of this water. Planning and implementing such projects requires understanding of the flow patterns and geochemistry of the target aquifers in addition to health and logistical constraints. This dissertation contains three studies of conditions for various phases of reclaimed water MAR projects in two different hydrogeologic and legal settings. Chapter 2 is a suitability analysis of land for reclaimed water MAR in the California's Central Valley in the context of the Sustainable Groundwater Management Act, which requires Groundwater Sustainability Agencies (GSA) to develop Groundwater Sustainability Plans (GSP). Land is scored in terms of the suitability of its soil and proximity to potential sources of reclaimed water. Inherently unsuitable land is excluded based on land cover and mandatory buffers for domestic wells. Backwards particle tracking is used to identify areas where recharged water would not meet minimum residence time criteria specified in California's water code, and these areas are also excluded. Land deemed suitable for reclaimed water MAR is compared to the land needed by GSAs in critically overdrafted basins to fulfil the recharge goals laid out in their GSPs. Potential recycled water availability is determined based on the discharge volumes of local water recycling and wastewater treatment plants. Under existing conditions, only 2 out of 29 GSAs have enough potentially suitable land to meet their recharge goals using sufficiently high-quality reclaimed water as a source, and none have enough high quality water, though the numbers increase if existing treatment plants can be upgraded. Chapter 3 details a tracer study and monitoring project conducted at the LOTT Clean Water Alliance's Hawks Prairie Reclaimed Water Ponds and Recharge Basins site in Thurston County, Washington. The study employs sulfur hexafluoride (SF6) and bromide (Br-) as added tracers to identify flow paths and determine residence times for reclaimed water recharged from the basins. As a low-solubility gas, the SF6 exhibits an exsolution – dissolution behavior according to Henry's law in the presence of air trapped within the soil and aquifer media, resulting in its retardation relative to the Br-. This retardation is used to calculate an air to water ratio of 10^-3 to 10^-2 along the flow paths of the recharge water. It is also demonstrated that the water follows several non-linear preferential pathways and that water can travel between the two uppermost aquifers in certain locations. Chapter 4 examines the broader geochemistry of groundwater in the upper and intermediate aquifer units of Thurston County, Washington, where the Hawks Prairie MAR site from chapter 3 is located. Groundwater is determined to be composed of concentrated meteoric water with additional solutes from mineral weathering. More evolved waters contain lower levels of oxygen with higher iron, manganese, and phosphorus contents. Groundwater may also be contaminated by seawater intrusion or septic effluent, resulting in elevated concentrations of chloride and, in the case of septic contamination, nitrate. This demonstrates the importance of familiarity with the local inputs that control groundwater quality in order to ensure compatibility with MAR. Together, these studies demonstrate the major phases and considerations of reclaimed water MAR projects. To be successful, projects must consider appropriate siting, transport conditions and geochemistry. With sufficient planning and attention to these details, reclaimed water MAR can be a valuable tool for ensuring water sustainability.