Soil Properties Below a Flood-Managed Aquifer Recharge Site on the Flood Plain of the Cosumnes River, Elk Grove, CA

Abstract:

Flood-managed aquifer recharge (flood-MAR) projects provide a unique opportunity to observe linked hydrologic processes involving surface water – groundwater interaction on flood plains. This study examines spatial differences in soil texture, and how the differences impact the future potential for groundwater recharge at a proposed flood- MAR site in the Central Valley of California. Soil samples were collected from the surface to 50 cm below ground surface (cm-bgs) at 10 cm intervals from 10 field locations to assess how well the mapped Natural Resources Conservation Service (NRCS) soil types matched field conditions, and to estimate important soil parameters such as hydraulic conductivity and infiltration capacity. Soil samples were analyzed for their grain size distribution using an optical, particle size analyzer. Grain size distributions show mainly a single-mode consisting of fine to medium sand (100 to 500 µm). Some samples contained smaller modes in the ranges of silt and clay. In general, the soil texture is consistent with designated NRCS soil types at the field site. The Kozeny-Carman equation was used to estimate the hydraulic conductivity of each sample, yielding values of 10^-8 to 2x10^-5 m/s. The estimated hydraulic conductivities of each sample were used to calculate the effective hydraulic conductivity of the upper 50 cm of soil at each sampling location. Calculated hydraulic conductivities were similar spatially, with no consistent trends with depth, although some sites showed an increase in conductivity by two to three orders of magnitude with depth. Infiltration rates were estimated for flood depths up to 2.5 m and for inverted water table depths up to 1 m, yielding values generally in the range of 1-2 cm/day. Spatial differences in calculated infiltration capacities were modest and showed no major trends. Relatively fine soils and low calculated infiltration rates suggest that flood-MAR benefits may be limited to 10-20 ac-ft/day per 100 acres of flooded landscape. However, these calculations neglect heterogeneities such as plant root tubules and animal burrows which are likely to increase mean infiltration rates and focus most infiltration to occur within small areas.