Water Quality of Drinking Water Supplies in Socorro, New Mexico

Socorro, a small town with a population of about 8,000, is located in central New Mexico along the Rio Grande within the Rio Grande rift, at the edge of an extensive volcanic field. Socorro has six sources of supply for drinking water. Two of these sources are thermal springs and four are wells ranging in depth from 97-500 ft. The water is not blended into one source for distribution, but rather each source serves as drinking water for those in the immediate area surrounding the well or spring. Each source was sampled and analyzed monthly over a 2-year period. The following parameters were determined and compared; temperature, pH, conductivity, TDS, hardness, alkalinity, Cl, SO₄, F, Br, NO₃, Na, K, Ca, Mg, SiO₂ As, Ba, Cd, Cr, Co, Cu, Fe, Pb, Li, Mn, Mo, Ni, Se, Sr, Ag, Th, U, and Zn. The monthly water usage from each source was also determined. High levels of arsenic (up to 42 ppb) and uranium (up to 55 ppb) occur naturally in the water sources, but not together in the same sources. Based on water quality parameters, the water may be grouped into three types with two sources in each type. Type 1. Low hardness (~70 ppm), low TDS (~240 ppm), no Fe (<5 ppb), no Mn (<5 ppb), very low U (3 ppb), and high As (~40 ppb). Type 2. High TDS (~700 ppm), high hardness (~260 ppm), high Fe (~100 ppb), high Mn (~650 ppb), low U (~6 ppb), and mid-level As (~24 ppb). Type 3. High TDS (~430 ppm), high hardness (~200 ppm), mid-level Fe (~50 ppb), mid-level Mn (~10 ppb), high U (25-55 ppb), and low As (~8 ppb). Considering the arsenic and uranium values, type 2 water appears to be a dilution of type 1 and type 3. This does not appear to be the case when comparing Fe, Mn, hardness, and TDS. Type 2 contains the highest Fe, Mn, TDS, and hardness. One possible explanation is that as type 1 and type 3 groundwater mixes and flows toward the river, some of it flows through areas higher in calcium carbonate and Fe and Mn mineralization producing the type 2 water which results in higher Fe, Mn, TDS, and hardness. Interestingly, the sources with the lowest TDS or the best quality water contain the highest arsenic levels. Four of the six sources contain arsenic above the USEPA's new MCL of 10 ppb.