Elsevier

Applied Geochemistry

Volume 24, Issue 3, March 2009, Pages 426-437
Applied Geochemistry

Influence of upwelling saline groundwater on iron and manganese cycling in the Rio Grande floodplain aquifer

https://doi.org/10.1016/j.apgeochem.2008.12.022Get rights and content

Abstract

Salinity contributions from upwelling groundwater significantly degrade water quality in the Rio Grande, a major source of water for the southwestern USA. This study considers the influence of this upwelling water on the geochemistry and microbiology of the Rio Grande floodplain alluvial aquifer. The composition of surface water, groundwater, and floodplain sediment samples collected from three transects in the Socorro Basin was examined. Terminal-restriction fragment length polymorphism (T-RFLP) was also used to examine microbial biomass samples. The distribution of salinity in the floodplain groundwater largely reflects the configuration of local groundwater flow and mixing of two major water sources, deeply-sourced saline groundwater and river water. Microbial populations in the shallow aquifer consume O2 and NO3- and serve to redistribute metal oxides from the saturated zone to locations of groundwater discharge at the surface and possibly near the water table. The upwelling saline groundwater affects floodplain microbial processes by transporting reduced metals and organic electron donors to the alluvial aquifer system. This enhances metal reduction in the saturated zone and ultimately metal oxidation at or near the surface. Geochemical modeling suggests that mixing of the saline groundwater with more dilute water in the floodplain creates conditions more favorable for metal oxidation to occur and thereby influences the distribution of metal oxides.

Introduction

The Rio Grande provides an invaluable source of water for agricultural, industrial, and domestic uses in the southwestern USA and northern Mexico. Water resources in this region are scarce, reflecting the semi-arid to arid climate. In fact, even in non-drought years, water demands exceed supplies in the basin and threaten adverse economic effects (Ward et al., 2006). Maintaining adequate water supplies along the river, therefore, is critical to sustaining the growth of urban and rural populations in the region.

Water quality in the Rio Grande decreases downstream in part due to salinization. The total dissolved solids (TDS) content of the water in the river increases from about 40 mg L−1 in Colorado to over 2000 mg L−1 near the US-Mexico border (Mills, 2003, Phillips et al., 2003). Much of this salinization results from concentrating processes including open water evaporation and riparian and agricultural transpiration. Recent studies using soluble ion ratios and isotopic tracers have shown that geological sources of salt added by upwelling old saline groundwater are also a major cause of salinization (Hogan et al., 2007, Mills, 2003, Phillips et al., 2003). Using a mass balance model, Mills (2003) calculated that influxes of saline groundwater and evapotranspiration account for about 30% and 55%, respectively, of the increase in the Cl content of Rio Grande water between its headwaters and the US-Mexico border.

Saline groundwater influxes appear to coincide with structural features in the underlying geologic formations. Along much of its route, the river flows along the Rio Grande rift, a north–south trending fault-bounded extensional structure defined by a series of deep alluvial-filled basins with high heat flow that are flanked by uplifted blocks on the east and west (Wilkins, 1998) (Fig. 1A). Unconsolidated to moderately consolidated sediments of the Santa Fe Group, which range to over 4200 m in thickness, account for the majority of fill in rift basins (Plummer et al., 2004). The basin fill is overlain by a thin layer of unconsolidated floodplain alluvial sediment, which ranges up to about 40 m in thickness (Plummer et al., 2004), and the basins are floored by dense low-permeability bedrock (Grauch et al., 2001). Salinity increases along the Rio Grande are most apparent at the transition zones between the rift basins, where Santa Fe Group deposits thin and the groundwater is forced to flow over structural highs in the underlying bedrock (Mills, 2003, Phillips et al., 2003). Saline influxes also likely occur due to upward leakage along major faults within and along the flanks of the basins (Newell et al., 2005, Newton, 2004, Plummer et al., 2004).

Rio Grande floodplain alluvium represents a zone of mixing between these upwelling mineralized waters and the river. A clear understanding of how the composition of surface water and shallow groundwater in the floodplain is affected is necessary for future management of this critical water resource. It may also provide insight into processes occurring in similar environments where deeply circulating groundwater mixes with surface water and shallow groundwater. The influence of upwelling saline groundwater on geochemistry and microbiology in the Rio Grande floodplain aquifer in the Socorro basin was investigated. The specific objectives were to (1) examine floodplain hydrologic and microbial processes using field observations and chemical and isotopic analyses of groundwater, surface water and sediment samples, (2) examine variation in the composition of the microbial community by analyzing 16S rRNA genes from samples of microbial biomass, and (3) evaluate the influence of groundwater upwelling on the geochemistry, microbiology and water quality in the floodplain aquifer system.

In riparian corridors where surface water and groundwater interact, such as the Rio Grande floodplain, microbial processes can control the availability of nutrients (Dahm et al., 1998, Duff and Triska, 1990), the distribution and form of organic pollutants (Conant et al., 2004), and the mobility of metals and trace elements (Harvey and Fuller, 1998) in both the surface water and groundwater. Microbial processes, therefore, likely have a significant influence on water quality in the Rio Grande floodplain.

In general, microorganisms conserve energy for growth and reproduction by catalyzing energetically favorable oxidation–reduction reactions. For many groups of microorganisms, electron-donating species in these reactions are derived from organic matter. Reduced inorganic species, including sulfide, NH4, reduced metals and H2, may also serve as energy sources when they are transported to less reducing areas from more reducing areas.

Respiring microorganisms couple oxidation of these energy sources to reduction of a terminal electron acceptor. Major electron acceptors available for respiration in the floodplain aquifer are O2, NO3, Mn, and Fe in oxide and oxyhydroxide minerals (hereafter referred to collectively as oxide minerals) and SO4. Where the availability of these electron acceptors is limited, methanogenic microorganisms may be active, which produce CH4 most commonly by either fermenting acetate or coupling H2 oxidation to CO2 reduction (Conrad, 1999).

The activity of respiring microorganisms often appears segregated into zones according to the electron acceptor in groundwater (Bethke et al., 2008, Champ et al., 1979). At least in part, this characteristic reflects competition between populations of microorganisms for electron donors (Lovley and Goodwin, 1988). Microbial populations that use electron acceptors that provide more energy may out-compete other populations that use less favorable electron acceptors. Oxygen respiration, for example, yields more energy than NO3 respiration. In a system closed to O2 and NO3, therefore, O2 would tend to be depleted first, followed by NO3. Where NO3 has been depleted, less favorable electron acceptors would then be used, including the Mn and Fe in oxide minerals and SO4.

The upwelling groundwater could influence microbial processes in the Rio Grande floodplain in a number of ways. It may cause variation in the temperature, pH, and salinity of the water in the floodplain, which are important environmental variables that can influence the composition and function of microbial populations (Madigan et al., 2003). Perhaps most importantly, the upwelling groundwater could increase the availability of electron donors used by microbial populations in the aquifer. Upwelling fluids in the Great African Rift, for example, have been shown to contribute H2 to Lake Kivu, fueling methanogenesis and resulting in high concentrations of CH4 in the lake water (Deuser et al., 1973). Inorganic energy sources, furthermore, are abundant in deeply circulating fluids upwelling at hydrothermal vents in the ocean (e.g., Jannasch and Mottl, 1985) and in many continental springs (e.g., Newell et al., 2005, Spear et al., 2005).

Water was examined at three sites in the northern end of the Socorro Basin (Fig. 1B). The northernmost site, SAC, is located near San Acacia, New Mexico and lies near the southern terminus of the Albuquerque Basin (or Middle Rio Grande Basin). Mills (2003) interpreted an increase in the Cl/Br ratio of Rio Grande water near San Acacia as evidence of upwelling saline groundwater there. Correspondingly, Anderholm (1987) observed groundwater with high Cl content near SAC and interpreted it to be upward-flowing groundwater from the southern end of the Albuquerque Basin. The two sites further south, ESC and BRN, are located near Escondida, New Mexico, and Brown Arroyo, respectively. Data collected by Mills, 2003, Newton, 2004 show no evidence of upwelling saline groundwater at ESC or BRN.

Primary sources of recharge for the study area include precipitation along mountain fronts, seepage from arroyos, floodplain irrigation canals, and the Rio Grande channel, and groundwater inflow from adjacent basins (Anderholm, 1987). During 2000–2001, measurements of seepage rates from the river channel made over a reach beginning at ESC and extending about 20 km south ranged from about 10,000 to 18,000 m3 day−1 km−1 (SSP&A, 2002). As already noted, groundwater inflow from the Albuquerque Basin occurs near SAC at the northern end of the Socorro Basin. Groundwater inflow from the La Jencia Basin supplies water to the western portion of the Socorro Basin (Anderholm, 1987) (Fig. 1).

Groundwater is removed from floodplain aquifers by pumping from wells and by evapotranspiration. Floodplain groundwater also discharges at the intersection of the water table with the low flow conveyance channel (LFCC). The LFCC lies just west of the river and consists of a 15 m wide rock-lined channel. The LFCC forces seepage loss from the Rio Grande because the LFCC is the topographic low in the floodplain. It was constructed during an extended drought in the 1950s to increase water delivery to Elephant Butte Reservoir in the southern end of the basin during periods of low flow (Moore and Anderholm, 2002). Diversions of Rio Grande water into the LFCC were stopped in 1985. Since then, the LFCC is fed by groundwater discharge and return flow from agricultural drains (Moore and Anderholm, 2002).

These natural and anthropogenic constraints on hydrology in the floodplain aquifer in the study area have led to the development of three groups of shallow groundwater flow-paths (Wilcox et al., 2007): (1) flow directed southeastward from the river, (2) flow southwestward from the river to the LFCC, and (3) flow directed southeastward from the western part of the basin to the LFCC. Regional groundwater flow is directed inward from the basin margins towards the Rio Grande and southward (Bexfield and Anderholm, 1997).

At each study site, transects of nested monitoring wells are aligned sub-parallel to groundwater flow in the floodplain alluvial aquifer. Each well nest contains a well completed at the water table (well A) and a well completed from about 8 to 12 m below the water table (well B). Individual wells are also present on the transects, including one completed at the contact between the unconsolidated alluvium and the Santa Fe Group at each site. The drillers logs from these relatively deep wells demonstrate that the floodplain alluvium is about 30 m thick in the field area (SSP&A, 2003).

Section snippets

Geochemistry

During February 16–22, 2006, groundwater was sampled west of the LFCC and between the Rio Grande and the LFCC at each transect. Eleven well nests were sampled and one individual well, an A well, at ESC. Surface water was also sampled in the river and the LFCC at each transect. A Proactive submersible DC driven pump was used to purge the wells and temperature, pH and conductivity were monitored using an Oakton 300 series meter. Samples were collected after pH, temperature, and conductivity

Hydrology

All spatial, hydrologic and chemical data for groundwater and surface water samples are provided in the online data repository. In general, hydraulic head measurements collected from wells west of the LFCC during sampling decrease southwards and towards the LFCC (Fig. 2). The head in wells between the Rio Grande and the LFCC at each transect was slightly higher than those immediately west of the LFCC. Hydraulic head varied by 2 cm or less, the assumed margin of error, within most well nests. The

Groundwater flow and mixing

The lateral variation in hydraulic head and temperature observed demonstrates that the pattern of groundwater flow in the floodplain when sampled was consistent with previous interpretations (e.g., Wilcox et al., 2007). West of the LFCC, the hydraulic head gradient favors shallow groundwater flow directed SE towards the LFCC. Between the river and the LFCC, the head gradient favors shallow groundwater flow directed SW towards the LFCC. Warmer groundwater and surface water at SAC compared to ESC

Summary and conclusions

In this study, the influence of saline groundwater upwelling on geochemistry and microbiology in the Rio Grande floodplain was examined. It was demonstrated that the controls on salinity observed on a large scale in the Rio Grande are also evident on a much smaller scale in shallow groundwater and surface water in the floodplain. A conceptual model was developed for floodplain microbial activity by taking into consideration variation in the composition of floodplain water, the pattern of local

Acknowledgements

We are grateful for assistance in molecular biology from Kendra Mitchell, Lydia Zeglin, and Laura Guest, in analytical chemistry from Abdul-Mehdi Ali, Johanna Blake, John Craig, and Kim Gugliotta, and in collecting samples from Ryan Jakubowski, Maceo Martinet, and Amy Luther. This manuscript benefited from thorough reviews by Dr. Clifford Dahm, Dr. Gary Smith, Dr. Peter McMahon, and an anonymous reviewer. This work was supported by student research funding from the New Mexico Geological Society

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    Present address: Geology Program, Western State College, Gunnison, CO 81231, USA.

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