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Research Highlights
Unlocking the Climate Archives
Sampling the upper catchments in the Elwha watershed, Washington
Within our continent’s aquifers lie archives of long-term climate changes describing precipitation recharge in watersheds. Aquifers act as filters that smooth out short-term climate fluctuations and record continental climate signals. Past temperature signals (paleotemperatures) are stored with dissolved noble (inert) gases in groundwater. Using noble gases as proxies, NRMRL hydrologists are sampling paleotemperatures in the Elwha watershed on the Olympic Peninsula in Washington State. The goal is to record paleotemperatures in the region in an effort to forecast the range of future shifts in recharge. Changing recharge patterns can result in severe consequences for many habitats, notably the salmon habitats of the Northwest. The Elwha watershed is the centerpiece of widespread efforts at salmon habitat restoration on behalf of tribal communities and declining regional fisheries.* BackgroundWhile no wells exist, nor can be drilled, in the Olympic National Park itself, the abundant springs and seeps in the area provide samples that are linked chemically and isotopically to recharge from the Olympic Mountains. When sampled for certain parameters, these springs provide a window into the regional aquifer’s recharge history. Recent advances in sampling technology allow micro-volume extractions from watershed sites and expedite the transport of dissolved gases for laboratory analysis. To provide a basis for paleotemperature estimates, researchers combine noble gas spectrometry with known solubility equilibrium constants for neon, argon, krypton and xenon. These inert gases dissolve in precipitation, infiltrating to the water table where, until air saturation is reached, they are dependent on temperature. A noble-gas temperature (NGT) is therefore a measure of the temperature at which groundwater equilibrated with the atmosphere during infiltration and it commonly corresponds to the mean air temperature. Measurement of the composition of each noble gas in a groundwater sample provides respective independent estimates of that recharge temperature. Solubility of each heavier noble gas is increasingly more sensitive to changes in temperature. Recharge temperatures are derived by extraction of air from the analyzed neon, argon, krypton, and xenon gas concentrations of groundwater samples. The key assumption is that the groundwater system remained closed since its recharge. Isotopes of these noble gases are measured to decipher if significant mixing, dispersion, or atmospheric contamination have occurred during their residence time. (Note that the present water temperature, such as at a hot or cold spring, does not disguise the recorded temperature of a dissolved noble gas entrapped since recharge.) A total of 63 springs or seeps have been sampled thus far in the Elwha watershed. In 13 of those samples, the recharge air temperatures varied from 8.2° to 12.4° centigrade over an interval of 900 years before the present. While cooler air temperatures prevailed, the present mean recharge air temperature was exceeded by much warmer air about 750 years ago. As more detailed maps of recharge air temperature changes are developed for younger and older recharged waters, scaled modeling of hydrologic impacts for salmon-spawning habitats will be forecasted. While many elements influence the decline of salmon runs, climate change and its relationship to stream temperatures are recognized as significant potential influences in the restoration of salmon populations in the Northwestern states. *In 2007, the governors of Washington, Oregon and California asked for federal disaster relief after a regional salmon fishery cancelled the commercial fishing season because of a critical decline in salmon populations. ContactJane Ice, NRMRL Office of Public Affairs (513) 569-7311
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