Identifying the sources of CO2 in carbonic springs in the Albuquerque-Belen Basin

Abstract

Understanding groundwater resources in the Albuquerque basin region requires an understanding of the geochemistry of carbonic springs and potential mixing among different water sources. Carbonic springs are defined as high PCO2 springs (PCO2 >10-1.8). This paper evaluates the sources of the high dissolved CO2 in these springs by summarizing the geochemistry of carbonic springs found along faults of the Rio Grande rift. Major ion chemistry helps define chemical characteristics of endogenic (deeply sourced) fluids entering the groundwater system and their variable mixing with epigenic waters (meteoric recharge). We also use major ion water chemistry analyses to estimate the percentage of CO2 derived from dissolution of carbonates (both rock and minerals) (Ccarb) in groundwater. We then use carbon isotopes to estimate the percentage of the remaining external CO2 (Cext) that was derived from organic material such as soil gas (Corg) plus endogenic CO2 that is from deeply derived sources (Cendo). The results show a high percentage of endogenic components in the west flank of the Rio Grande rift spring waters with a range of 17.5 74.8% Cendo (mean value of 55.3%±19.8%). We analyzed dissolved gases to illustrate a spectrum of mixing between air and air-saturated groundwater with helium-rich deeply sourced fluids. The high endogenic CO2 in springs and travertines that occur within the Rio Grande rift at San Acacia and along much of the western rift faults from Socorro to I-40 is interpreted to reflect degassing of magmatic volatiles from the Socorro magma body. The wide distribution of springs suggests that similar waters may be cryptically entering Santa Fe Group aquifers from below and affecting water quality by adding salinity and trace metals as well as deeply sourced volatiles. These endogenic inputs are tepid (up to 26°C) and have geochemical similarities to geothermal waters. The variation in hydrochemistry of the Albuquerque basin can be attributed in part to mixing of endogenic fluids with other groundwater and has implications for future management of groundwater resources. 419