USGS Groundwater Information
Heat as a tracer for examining enhanced recharge processes along the Russian River, CA
By James E. Constantz
Ground water adjacent to the Russian River in Sonoma County, CA, is the primary source of drinking water pumped to Sonoma County Water Agency (SCWA) treatment facilities. This ground-water resource is superior to direct surface-water options, because experience has demonstrated that water extracted from the alluvial aquifer requires substantially less treatment than water extracted directly from the river, tributaries, or reservoirs. From late spring to early winter, SCWA erects an inflatable dam to raise the river stage and passively recharge the alluvial aquifer. The raised stage permits diversion of river water to a series of recharge ponds located upstream of the dam along the river. This results in enhanced extraction efficiency from water-supply wells situated along this reach of river. Production from these wells typically is reduced by 75% when the dam and recharge ponds are out of operation. Emerging issues, including fish habitat concerns and optimization of water resources management, indicate that a quantitative model should be developed to accurately represent river/ground-water exchanges in the region of the watershed encompassing the inflatable dam. Improved scheduling of dam and recharge pond operations, as well as supply well pumping patterns would be aided by the development of a proven ground-water model for this region of the watershed. Successful development of a model requires that key hydraulic parameters be identified, including the spatial and temporal pattern of river conductance. Several tools are available to estimate these hydraulic parameters, such as pumping tests and chemical tracers. Some pumping tests have been performed; however, introduced chemical tracers are not an option for the Russian River, due to environmental and esthetic concerns.
As an alternative to chemical tracers, the natural variation in river temperature affords the opportunity to use heat as a tracer of stream/ground-water exchanges in this reach of the Russian River. Earlier work successfully used heat as a tracer of ground-water recharge at various locations throughout the USA. Along the eastern seaboard, Lapham  used annual temperature records from deep observation wells to identify rates of vertical water flux in several streams, based on analytical solutions reported in earlier work [Lapham, 1988]. In the Rio Grande near Albuquerque, NM, Bartolino and Niswonger [1999 ] used a similar approach with a USGS ground-water model, VS2DH, [Healy and Ronan, 1996]. They matched simulated temperature to observed temperature, yielding predicted estimates of deep streambed fluxes and spatial-averaged hydraulic conductivities. Thomas et al.  used the same model to demonstrate that diurnal temperature patterns in shallow sediment could be successful in identifying shallower streambed conductance values at several locations along a reach of the Santa Fe River, NM.
For the Russian River, approximately 25 observation wells were instrumented for water-levels and ground-water temperatures, to compare with river stage and surface-water temperatures. Observed temperatures are being used to optimize simulated temperature from VS2DH, to predict the hydraulic conductivity at specific locations along this reach of the river. Early results identify a gradual decline of the streambed hydraulic conductivity upstream of the dam over the summer, which is probably due to accumulation of sediment.
The use of heat as a tracer to estimate percolation rates is also ongoing at the recharge pond facility in the vicinity of the inflatable dam. Again it is undesirable to use chemical tracers in this setting, so that natural variations in pond temperature should afford the opportunity to examine recharge characteristics. Earlier work by Nightingale [ 1975] and Constantz et al.  suggests that heat could potentially be an excellent indication of recharge rates. Simulation results demonstrate that shallow temperature monitoring is useful for ponds operated in a low-stage mode, where diurnal temperature signals could be expected to be significant. For recharge ponds with greater stages, deeper sediment temperature monitoring is necessary to capture the annual temperature signal.
The recharge ponds near the Russian River are operated with a stage of only 0.4 to 0.8 m, indicating that shallower sediments temperature should be monitored for diurnal analysis of heat and water transport beneath this facility. An intriguing thermal dissimilarity was observed for passive recharge from the river compared with the artificial recharge from the ponds. The use of heat as a tracer requires a accurate estimate of the upper thermal boundary condition. Observations on the Russian River have demonstrated that for virtually identical micrometeorological conditions, the sediment temperature profiles beneath the river are distinctly different than those beneath a standing body of water. For the river, water temperature and the surface-sediment temperature are comparable; however, this is not the case for shallow recharge ponds. During daylight, sediment surfaces have been observed to be several degrees greater than the standing water column in the recharge ponds, because solar energy is absorbed at a rate faster than its removal rate. Temperature equipment was installed below Recharge Pond #3 at depths between the surface and 1.5 m and logged on a 30 minute frequency. This data is being used to estimate temporal patterns of percolation beneath the recharge basin on a daily basis. Observed temperatures show an increasing thermal gradient between the surface sediments and sediments at depth, over the duration of each recharge period. Increased vertical gradient indicates reduced advection of heat to deeper sediments, as a result of reduced recharge rates. Monitoring of this type of thermal information can be utilized to schedule surface-water routing during ponding and drying cycles for the recharge ponds.
Recently, a study was initiated to compare sediment-temperature-based measurements of seepage loss with surface-water-based estimates. In both the river and the recharge facility, shallow temperature profiles are compared to seepage measurements, derived from seepage meters temporary installations near temperature equipment. Initial comparisons of temperature-based and seepage-meter-based percolation rates are very encouraging, with good agreement for both the river sites and Recharge Pond #3. In the future, continuous monitoring of ground-water-temperatures may lead to the capability to examine real-time trends in hydraulic clogging caused by sedimentation, and thus the ability to monitor real-time changes in ground-water recharge. Finally, this type of spatial and temporal parameter characterization will be useful in the planned construction of a MODFLOW model for this region of the Russian River, which is intended to evaluate the effects of various future scenarios on the water resources of Sonoma County.
Bartolino, J.R., and R. Niswonger, Numerical simulations of vertical ground-water fluxes of the Rio Grande from ground-water temperature profiles, Central New Mexico, U.S. Geological Survey, Water Resour. Invest. Rep. 99-42-12, pp.34, 1999.
Constantz, J., R. Niswonger, and A.E. Stewart, 1999, The use of heat as a tracer to estimate recharge beneath streams and artificial recharge ponds, in (ed.) R.D. Bartlett, Artificial Recharge of Groundwater, Amer. Soc. Civil. Engin., 193-203.
Healy, R.W., and A.D. Ronan, 1996. Documentation of computer program VS2DH for simulation of energy transport in variably saturated porous media, U.S. Geological Survey Water Resour. Invest. Rep. 96-4230, 36pp.
Lapham, W.W., Conductive and Convective Heat Transfer Near Stream, Ph.D. Dissertation, University of Arizona, Tucson, Arizona, pp. 315, 1988.
Lapham, W.W., Use of temperature profiles beneath streams to determine rates of vertical ground-water flow and vertical hydraulic conductivity , U.S. Geol. Surv. Water Supply Pap. 2337, 35ppp. 1989.
Nightingale, H.I., 1975. Groundwater recharge rates from thermometry, Ground Water, vol. 18(4), 340-344.
Thomas, C.L., A.E. Stewart, and J. Constantz, 2000, Comparison of methods to determine infiltration and percolation rates along a reach of the Santa Fe River near La Bajada, New Mexico, U.S.G.S. Water Resour. Invest. Rep. 00-4141, pp.31.
In George R. Aiken and Eve L. Kuniansky, editors, 2002, U.S. Geological Survey Artificial Recharge Workshop Proceedings, Sacramento, California, April 2-4, 2002: USGS Open-File Report 02-89
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