New Mexico Geological Society Annual Spring Meeting — Abstracts

"Eldorado" describes residential subdivisions created over the last thirty years by AMREP, also the developer of Rio Rancho. This city of gold covers about 40 square miles, occupying the piedmont plain of the San Marcos watershed within the Santa Fe embayment of the Espanola basin. To address water supply concerns, for several years the Eldorado area has been subject to a county moratorium on new subdivisions. Eldorado shares the water resources with Lamy, Canda de los Alamos, Cerrillos and numerous rural residents. The hydrogeology is complex, and abundant groundwater has not been identified. Groundwater is being mined. All Residents utilize septic tanks for liquid waste disposal. "Downstream" groundwater users rely on through-flow within the local hydrologic system.

AMREP created El Dorado Utilities, to permit the creation of smaller lots. Water service to 2300 users is provided by 15 wells, including the Lamy well beside Galisteo Creek. The Lamy well has historically produced one third of the total water used, and is drought sensitive. Overall, groundwater production grew exponentially through the 1980s and early '90s. Also during this period hundreds of domestic wells were drilled.

Groundwater recharge is relative modest in the low foothills of the Sangre de Cristos. However, significant stream losses and potential recharge are inferred form a watershed wide storm water model. The storm water model also suggests that runoff and recharge are extremely sensitive to the range condition, believed to have degraded since occupation by livestock. Discharge occurs by storm runoff, underflow and spring flow to Galisteo Creek, and by pumping. Upward of 1000 acre feet per year is now being appropriated in the watershed.

Beneath the piedmont, the principal aquifer is the Ancha Formation, which uncomformably overlies very complex geology, including volcanics and Mesozoic and Paleozoic formations. Some areas have modest groundwater availability ;and water wells have been drilled to 1000 feet. The area is being studied using geophysics and new surface geologic mapping. A conceptual model of the hydrologic system places most groundwater flow in the shallow aquifer, although some interesting deep aquifer possibilities are suggested.

Future water quality and supply issues link Eldorado and its neighbors within the watershed. Although desirable, it is unreasonable to expect to determine a 'sustainable groundwater yield' within the watershed because of the geOlogic complexity and vagaries of New Mexico climate and recharge. Ongoing studies may help extend the resource, but the equities of all users will someday require a broader sharing of and caring for the resource in a unified groundwater management scheme.