USE OF DEEP (AND SHALLOW) PASSIVE CAPILLARY SAMPLERS (PCAPS) TO EXAMINE SOIL WATER MOVEMENT AND GROUNDWATER RECHARGE PROCESSES

Soil waters from three nearby sites in the Sierra Nevada were collected for isotopic analysis using passive capillary samplers (PCAPS). The PCAPS were placed at depths of 0.3, 1.0, and 1.5 m below ground surface at each site (previous applications of PCAPS have used them only at relatively shallow depths). In addition to the PCAPS, at each of the three sites, bulk precipitation was collected, and a special collector was employed to collect infiltration from snowmelt. Samples were collected seasonally, integrating the summer/early fall periods and the winter/early spring periods.

One site was located at 2350 masl; the other two were located at 2114 masl. The two sites as 2114 masl were adjacent to each other, but one was in an open, unshaded area, and the other in an area shaded by vegetation.

At all three sites, isotopically light water moved down to at least 1.5 m bgs during/after the spring snowmelt period. However, this water did not displace the existing soil water; the isotopic data show that mixing occurred between soil water in place prior to the snowmelt period and the snowmelt itself. In most cases, the amount of mixing decreased as a function of depth, meaning that at less water penetrates the soil column as depth increases.

During the summer and early fall, soil waters became isotopically enriched. In some cases, evaporation alone could explain the enrichment, but in other cases, some combination of evaporation and mixing with summer precipitation could account for the observed isotopic evolution. Other waters’ isotopic evolution showed that at least some evaporation must have occurred, as mixing with warm-season precipitation alone could not explain the observed change.

Comparison of adjacent shaded and unshaded sites showed that the shaded site had a great difference between the winter bulk precipitation and snowmelt isotopic signatures. Some combination of isotopic alteration during snow interception and/or increased opportunity for isotopic alteration due to shading and increased snow persistence explains this difference.

The spring snowmelt pulse was able to drive greater volumes of water to deeper depths than were individual summer precipitation events. This suggests that in a warming climate, a shift from snow to rain could result in less groundwater recharge.