Identifying origins of and pathways for spring waters in a semiarid basin using He, Sr, and C isotopes: Cuatrocienegas Basin, Mexico

Abstract

He, C, and Sr isotopes are used to infer spring sources in a water-stressed area. Spring-water origins and pathways in the Cuatrociénegas Basin are revealed by linking structure and geochemistry via regionally extensive fault networks. This study presents the fi rst dissolved noble gas and He isotopic data from northeastern Mexico. Basementinvolved faults with complex reactivation histories are important in northeastern Mexico tectonics and affect hydrogeologic systems. The importance of faults as conduits for northeastern Mexico volcanism is recognized, but connections between faulting and the hydrogeologic system have not been extensively investigated. This research tests the hypothesis that Cuatrociénegas Basin springs are divided into two general classes based upon discharge properties: (1) regional carbonate aquifer discharge (mesogenic) mixed with contributions from deeply sourced (endogenic) fl uids containing 3 He and CO 2 from the mantle that ascend along basement-involved faults; and (2) carbonate aquifer discharge mixed with locally recharged (epigenic) mountain precipi tation. Carbonate and/or evaporite dissolution is indicated by Ca-SO 4 hydrochemical facies. He isotopes range from 0.89 to 1.85 R A (R A is the 3 He/ 4 He of air, 1.4 × 10 -6 ) and have minimal 3 H, from which it is inferred that basement-involved faults permit degassing of mantle-derived He (to 23% of the total He) and CO 2 ( pCO 2 ≤ 10 -1 atm). Mantle degassing is compatible with the thinned North American lithosphere, as shown in tomographic images. Sr isotopes in both Cuatrociénegas Basin springs and spring-deposited travertine ( 87 Sr/ 86 Sr = 0.707428-0.707468) indicate that carbonate rocks of the regional Cupido aquifer ( 87 Sr/ 86 Sr = 0.7072-0.7076) are the main source of Sr. Rock-water interactions with mafi c volcanic rocks ( 87 Sr/ 86 Sr = 0.70333-0.70359) are not inferred to be an important process. Groundwater-dissolved inorganic C origins are modeled using major elements and C isotopes. C isotope data show that ~30% ± 22% of CO 2 in spring water is derived from dissolution of aquifer carbonates (C carb = 30%), 24% ± 16% is from soil gas and other organic sources (C org = 24%), and 46% ± 33% is from deep sources [C endo (endogenic crust and mantle) = 46%]. This study demonstrates the presence of mantlederived 3 He and deeply sourced CO 2 that ascend along basement-penetrating faults and mix with Cupido aquifer groundwater before discharging in Cuatrociénegas Basin springs.

Understanding the hydrogeology and surface flow in the Cuatrociénegas Basin (NE Mexico) using stable isotopes

“…Waters issuing from the western springline ( Fig. 2) and high discharge springs in the CCB support the wetland ecosystem located around the piedmont of the Sierra San Marcos, and are the same as those used to irrigate farms in the region (Wolaver et al 2013). Thus, areas W(a) and (b) were of particular importance due to their proximity to the western springline of the Sampling sub-region (Fig.…”

“…Recharge elevation has been suggested as a reason for these differences (Johannesson et al 2004;Rodriguez et al 2005) with estimates in the region of -0.48‰/100 m for δ 18 O (Rodriguez et al 2005). However, residence times >60 yr (Wolaver et al 2013) and the well mixed nature of the CCB groundwater indicated by adherence to the LEL (Fig. 3a) suggests that recharge elevation estimates may be inaccurate (Blasch and Bryson, 2007).…”

“…Thus, areas W(a) and (b) were of particular importance due to their proximity to the western springline of the Sampling sub-region (Fig. 2) Hypothesised flow systems Sub-region description Area W(a) Churince system (Minckley, 1969;Evans, 2005) A mixture of hot pools and a cool lake (Laguna Grande) in close proximity to the fault zone of the Sierra San Marcos, the main discharge zone of the CupidoAurora aquifer in the CCB (Johannesson et al 2004;Wolaver et al 2013) Area W(b)…”

“…comm., January 2010). Recent estimates also indicate a ~1m/yr drop in the water table levels of the surrounding basins since extraction began (Wolaver et al 2013). …”

Filling a Geographical Gap: New Paleoecological Reconstructions From the Desert Southwest, USA

“…Groundwater lags have been shown to have long residence times of decades to centuries (Rodríguez et al, 2005;Wolaver et al, 2013) In the Southwest, winter and spring precipitation is enhanced during El Niño events (Heyer et al, 2017). Scanlon et al (2006) indicated that periods of frequent El Niño events result in three times as much groundwater recharge in the desert Southwest as compared to periods of frequent La Niña events.…”