Abstract
During the dry months of the water year in Mediterranean climates, groundwater influx is essential to perennial streams for sustaining ecosystem health and regulating water temperature. Predicted earlier peak flow due to climate change may result in decreased baseflow and the transformation of perennial streams to intermittent streams. In this study, naturally occurring radon-222 (222Rn) was used as a tracer of groundwater influx to Martis Creek, a subalpine stream near Lake Tahoe, CA. Groundwater 222Rn is estimated based on measurements of 222Rn activity in nearby deep wells and springs. To determine the degassing constant (needed for quantification of water and gas flux), an extrinsic tracer, xenon (Xe), was introduced to the stream and monitored at eight downstream locations. The degassing constant for 222Rn is based on the degassing constant for Xe, and was determined to be 1.9–9.0 m/day. Applying a simple model in which stream 222Rn activity is a balance between the main 222Rn source (groundwater) and sink (volatilization), the influx in reaches of the upstream portion of Martis Creek was calculated to be <1 to 15 m3/day/m, which cumulatively constitutes a significant portion of the stream discharge. Experiments constraining 222Rn emanation from hyporheic zone sediments suggest that this should be considered a maximum rate of influx. Groundwater influx is typically difficult to identify and quantify, and the method employed here is useful for identifying locations for focused stream flow measurements, for formulating a water budget, and for quantifying streamwater–groundwater interaction.
Highlights
Headwater basins are recognized as being critically important for generating runoff that is captured in reservoirs and used for irrigation and municipal water supplies
Snowpack in the Sierra Nevada of California allows for slow melting and gradual groundwater recharge in basins; as more precipitation occurs as rain, more limited opportunity for groundwater recharge is likely to cause increased run-off as overland flow [1,2,3,4,5]
We identified reaches of Martis Creek with groundwater discharge by measuring the concentration of naturally occurring 222 Rn and introduced xenon (Xe)
Summary
Headwater basins are recognized as being critically important for generating runoff that is captured in reservoirs and used for irrigation and municipal water supplies. Precipitation in subalpine regions will occur more frequently as rain rather than snow, which could have drastic impacts on stream flow and on groundwater recharge. Snowpack in the Sierra Nevada of California allows for slow melting and gradual groundwater recharge in basins; as more precipitation occurs as rain, more limited opportunity for groundwater recharge is likely to cause increased run-off as overland flow [1,2,3,4,5]. Groundwater is essential to the area as it provides baseflow to Martis Creek during the dry summer months, which is critically important for maintaining stream ecosystem health. Discharge that ends earlier in the summer or fall as a result of climate change or of groundwater pumping that continues into the summer and fall will put stress on the baseflow of the stream. Groundwater discharge to the stream moderates stream temperature, especially in the late summer and fall, which is essential to the viability of the fish population in the stream [6,7,8,9,10]
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