Untitled DocumentRio Grande solute balance and DSMs
Macro Theme Area:
River Systems [Project ID: R04]
PI:
James Hogan
CO-PI(s):
Fred Phillips
Basin focus:
Rio Grande
Specific area in
basin /
field sites:
Rio Grande from headwaters in Colorado to Ft. Quitman, Texas
Summary/Goals: Issues of water quality often limit water resources availability in semiarid and arid regions. The Rio Grande, with its elevated salinity levels near El Paso, arsenic concentrations near the EPA drinking water standard, and significant nutrient loading from the Albuquerque wastewater treatment plan leading to anoxic bottom waters in Elephant Butte Reservoir, illustrates this fact well. This project seeks to identify and quantify the solutes along the Rio Grande in order to develop improved water management strategies. There are essentially two phases to this project. The first focuses on developing a conceptual understanding of the sources and sinks of solutes along the Rio Grande. From 2000-2006 we conducted biannual synoptic sampling of the Rio Grande from its headwaters in Colorado to ~150 km south of El Paso, Texas. This period coincides with a severe regional drought, allowing us to further basin address response to periods of increasing water stress. We have employed multiple environmental tracers to help identify dominant hydrological processes and the causes of salinization. Briefly, O and H isotopic results indicate that runoff from high-elevation areas in Colorado and northern New Mexico -primarily as snowmelt - is the dominant source of river water. Salinity increases along the Rio Grande are not a simple function of distance downriver, but rather occurred in a series of steps localized at the southern ends of the sedimentary basins. Isotopic data have been used to fingerprint and quantify the flux of brines discharging from these sedimentary basins (these results were recently published in Geology). The second phase transfers this understanding into a dynamic simulation model of the river system. This model will couple salinity and nutrient sources and sinks with a river systems water balance (including riparian and agriculture ET). This model will then allow decision makers to evaluate the impact of alternative management scenarios on water quality, including riparian restoration and evaluating the potential water quality impacts of a water banking system.
Nutrient research related to this project is described under project R12.
Activities and outcomes during past year:
The summer of 2006 represented one of the wettest monsoons on record providing a full range of hydrologic conditions during our sampling and our isotopic data indicates a significant component of local monsoon precipitation for the middle and lower Rio Grande valleys
For phase two of the project, Heather Lacey (for her MS thesis) designed a model using a system dynamics software program to analyze historical chloride concentration and discharge data Rio Grande chloride data. Modeling of Elephant Butte Reservoir revealed a significant, unexplained source of chloride in the reservoir, which contributed 114,500 tons of chloride between 1979 and 2004, the majority of which flowed from the reservoir as the reservoir level was dropping. Modeling of the river from Lobatos, CO to El Paso, TX revealed sources of chloride at San Acacia, the Low Flow Conveyance Channel, and between Elephant Butte Dam and El Paso, all of which contributed on average at least 16 % of the total chloride load in each reach. The chloride additions at these locations were not constant over the historical record, but varied over time and these variations were weakly correlated with discharge and brine inflows at San Acacia and El Paso showed a weak negative correlation with the drought index. The most significant source of chloride cumulatively over the length of the river is wastewater. Of nearly equal importance is brine inflows which are added to the river at San Acacia, the Low Flow Conveyance Channel, and El Paso, the largest of these additions being at the Low Flow Conveyance Channel. Tributaries are also a significant source of chloride to the Rio Grande, on the upper and middle Rio Grande.
The chloride model of the Rio Grande was also modified to simulate river conditions similar to those that existed before the construction of dams and extensive irrigation based on data collected by Stabler [1911] between 1905 and 1907. This simulation showed that, even without evaporation from Elephant Butte Dam and with little irrigated agriculture, the chloride concentration and chloride load in the Rio Grande increased significantly between San Marcial and El Paso. This model showed that brine inflows between San Marcial and El Paso between 1905 and 1907 were on average 7886 tons/mo, much higher the average of 1705 tons/mo for more recent times. Furthermore, the pre-Elephant Butte Reservoir model demonstrated that a large portion of the increase in both chloride load and chloride concentration between San Marcial and El Paso (28% and 70% respectively) may be attributed to changes in the system such as construction of reservoirs and increased agriculture.
Work on reactive solutes has been three-fold. First we have assembled a geochemical database that includes available historic and current geochemical data from Stabler (1905-1907), Wilcox (1934-1953), and the USGS for 14 river locations from Del Norte to El Paso. Second we used this database to investigate variations in river chemistry through time finding higher concentrations for all major anions and cations in the river historically than for the current period (1975-2004). Furthermore, under current conditions magnesium, calcium and bicarbonate solute loads (solute concentration multiplied by discharge) decrease below Elephant Butte Reservoir, possibly due to the settling out of carbonates within the reservoir. Finally, we have investigated the effects of irrigation practices through analysis of irrigation and ground water samples coupled with a water mass balance for a representative site (Lemitar, NM). Our results suggest that irrigational practices send additional amounts of solutes to the Rio Grande each year (200 kg/ha/yr of calcium, 1300 kg/ha/yr bicarbonate, 100 kg/ha/yr of magnesium, 80 kg/ha/yr of sodium and 240 kg/ha/yr of sulfate); NETPATH analysis indicates that this is likely due to the dissolution of calcite, dolomite and gypsum as well as cation exchange reactions.
Plans for the upcoming year:
We have completed much of the research focused on the conceptual understanding of solute sources and sinks. As SAHRA's funding is ramping down we discontinued synoptic sampling. Given the length of record and the hydrologic extremes represented in our dataset there is little to be gained by additional sampling. During 2008 our focus will be on synthesis our data and in particular evaluating the role of climate variability on the hydrologic and geochemical processes occurring along the Rio Grande. At least one addition publication, focused on O/H isotope results, will be prepared during this year.
Additional work in 2008 will focus on understanding the processes that affect the reactive solutes and continued development of our model to evaluate future climatic and management scenarios in order to plan for the future water needs of the basin. The trends observed between groundwater and irrigation water at the Lemitar site might bring insight to chemical compositional differences between data from the 1900's [Stabler] and more recent times (1990's). Most pronounced in the anion data, the 1900 data has a similar composition to current irrigation water, whereas the chemical composition of the 1990 data, plots closer to current groundwater on a piper diagram. The historical water quality data is not sufficient to support a NETPATH analysis along the whole river which may help understand changes in reactive solute transport. To undertake this analysis we are reevaluating the data quality and frequency, and filling in missing data by regression, where possible. We are also conducted a "winnowing" statistical experiment to evaluate how much data are adequate and when the data become too few.
Organization Involvement:
IAEA has organized a coordinated research program (CRP) on "Design Criteria for a Network to Monitor Isotope Composition of Runoff in Large Rivers"
Shared Resources / Joint Activities:
SAHRA, through the Rio Grande solute balances project, has participated in this CRP since 2001. Data collection activities will be coordinated so that intercomparison of large river basins may be possible.
Organization Involvement:
The New Mexico Interstate Stream Commission is charged with managing New Mexico's water resources. The NM-ISC has recently funded two projects that are closely tied to the Rio Grande salinity study. One project (led by Rob Bowman at NMT) is studying GW-SW interactions along the Rio Grande south of Albuquerque.
Shared Resources / Joint Activities:
We are currently working with both these groups on a limited basis. Mainly we are sharing some data and samples. We have analyzed several well samples collected through these projects to quantify our saline groundwater end member.
