UC Santa Cruz

Groundwater is a critical resource facing declining supply and quality around the world. Distributed stormwater collection coupled with managed aquifer recharge (DSC-MAR) is a developing strategy to enhance groundwater recharge by collecting excess hillslope runoff in infiltration basins. DSC-MAR projects provide opportunities to learn about the inter-related hydrologic processes of runoff generation, infiltration, and recharge in the context of sustainable groundwater management. The three chapters presented here use interdisciplinary approaches to study these processes at multiple scales, including (1) regional computer models evaluating the impact of shifting climate and land use on runoff generation and recharge, (2) multi-year field observations quantifying the dynamics of runoff generation and infiltration, and (3) field-based experiments linking hydrologic, geochemical, and microbial processes during rapid infiltration. In Chapter 1, models suggest that urban and agricultural development reduce the threshold for runoff generation relative to pre-development land use, resulting in up to 2.3 times as much runoff and less recharge basin-wide. When incorporated into an MAR suitability map combining soil, bedrock, and aquifer properties, model results demonstrate that DSC-MAR projects could be effective in many locations during a range of climate scenarios. Field results from Chapter 2 show that a pilot DSC-MAR project was successful, collecting and infiltrating 5.3 × 105 m3 (426 ac-ft) over six years, including an extended regional drought. Runoff generation was sensitive to sub-daily storm frequency, duration, and intensity; observed infiltration rates varied widely in space and time; and 8.2 × 105 kg of fine-grained sediment accumulated in the infiltration basin over three years, likely reducing soil infiltration capacity. In Chapter 3, soils below a horizontal permeable reactive barrier (PRB) made of woodchips had greater nitrate removal (1.5 g/m2/day NO3-N) than un-amended native soils (0.09 g/m2/day NO3-N), despite rapid infiltration up to 1.9 m/d. Many putative denitrifying bacteria were enhanced in soils below the PRB. A PRB seemed to create favorable conditions for denitrification in underlying soils and could be applied to improve water quality during DSC-MAR. Collectively, the results of these studies advance our understanding of fundamental hydrologic processes and inform strategies to improve groundwater supply and quality.