Water Quality of Drinking Water Supplies in Socorro, New Mexico

Lynn Brandvold, New Mexico Bureau of Geology and Mineral Resources, New Mexico Tech, Socorro, NM 87801

ABSTRACT

Socorro, a small town with a population of about 9,000, 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, SiO4, 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.

The municipal water sources in Socorro can be classified into two main groups based on water type and Ca/Sr ratios. The northernmost sources (Eagle Picher, Olsen, and Schools of Mines wells) fall into a Ca-Na-HCO3 group and the southernmost sources (Industrial Well and the two thermal springs) fall into a Na-Ca-HCO3 group.

High levels of arsenic (up to 40 ppb) and uranium (up to 55 ppb) occur naturally in the water sources, but not together in the same sources. The USEPA has recently set a controversial MCL for arsenic in drinking water of 10 ppb. Arsenic levels in 4 of Socorro's 6 sources are above the 10 ppb level and will require treatment. The USEPA is also considering an MCL for uranium of 20 ppb. Two of Socorro's sources are above this limit. Based on As levels the sources can be classified into three groups; a high As (Socorro and Sedillo Springs), mid-level As (Industrial and School of Mines wells) and low As (Olsen and Eagle Picher wells). The Industrial Well water with its lower As and increased Na and Cl appears to be a result of further mixing of the spring water at depth with a Na-Cl rich water that doesnít contain much As. The other three wells are the same water type as that found in La Jencia Basin recharge area indicating they have probably flowed through faults in the Socorro Mountain Block and havenít mixed with deeper waters.

Iron and manganese values are high in three of the wells and vary with time more than any other elements. Neither of these elements are a public health problem but are rather nuisance elements. In their reduced form both are soluble, but when they become oxidized during pumping and chlorine disinfection they become insoluble. This is a particular nuisance when laundering white clothes with detergent that contains a bleaching agent.

Interestingly, the sources with the lowest TDS or the best quality water contain the highest arsenic levels and the only two sources that don't contain arsenic above 10 ppb, contain uranium above the 20 ppb proposed USEPA limit.