Untitled DocumentIsotopic quantification of mountain front recharge in southeast Arizona
Macro Theme Area:
Basin Scale Water Balance [Project ID: B13]
PI:
James Hogan
CO-PI(s):
Brenda Ekwurzel
Basin focus:
San Pedro
Specific area in
basin /
field sites:
Two transects one along the Huachuca Mnt Front the other up Ramsey Canyon to Carr Peak
Summary/Goals: The sustainable management of water resources, to meet both increasing human demands and maintain groundwater-fed riparian ecosystems, is predicated on accurate knowledge of natural recharge rates. Quantifying natural recharge is crucial for long-term sustainability, for developing an effective artificial recharge strategy, and for establishing and maintaining riparian conservation goals. In this project we employ isotopic tracers to investigate mountain front recharge (MFR), an important if not dominant recharge pathway for alluvial basin aquifers in Arizona. The goal is to improve mountain front recharge (MFR) estimates through an improved conceptual understanding of recharge processes and by providing an independent constraint on recharge rates.
Our approach is to use stable isotopes combined with the recharge temperature/elevation derived from noble gas concentration for source water identification and use tritium-helium and radiocarbon isotopes for residence time determinations. Specific questions addressed in this research are: a) How is recharge partitioned between winter and monsoon (summer) precipitation?; b) What percentage of recharge occurs through the mountain block versus through stream channels at the mountain front?; c) What are the rates of mountain front recharge and mountain block recharge?; and d) What is the elevational gradient in recharge and how is it affected by bedrock and vegetation type?
This is a TRIF funded project associated with the SAHRA program. This project when combined with "Quantifying regional groundwater contributions to baseflow of the San Pedro River (R05)" will provide a basin-scale view of runoff, recharge and groundwater flow processes from an isotopic perspective.
Activities and outcomes during past year:
2005 Report: The last 12 months have been focused on analysis and publication of results from the project. Results have been presented at two national meetings as well as to the USPP. Results of the project can be summarized as follows:
Use of geochemical tracers has improved our conceptual and quantitative understanding of mountain system recharge (MSR) in the Upper San Pedro Basin, Arizona. These isotopic, major anion, and noble gas tracers resolved the location, rate and seasonality of recharge as well as groundwater flowpaths and residence times. Detectable anthropogenic 14C and tritium revealed less than 50 years residence times for the fast fraction of flow on the mountain block and in riparian areas. Maximum 14C residence times of over 10,000 years occurred for basin groundwater entering the San Pedro River riparian area. Groundwater fluxes determined from 14C imply MSR rates between 2 106 m3/yr and 9 106 m3/yr. Stable isotopic signatures indicate that MSR has a 40-90% contribution from winter precipitation and a 10-60% contribution from summer precipitation. Geochemical data is recommended to help calibrate groundwater flow models and substantially reduce uncertainty in the estimated MSR rate.
Results from sulfur isotopic and noble gas analysis proved less than conclusive and did not substantially enhance our understanding of MSR over the other isotopic data.
2004 Report: Over the last twelve months we have completed our field sampling and a substantial portion of the laboratory analyses. Major accomplishments include obtaining all the necessary permission for sampling wells on land belonging to a wide variety of government, corporate, and private entities; collecting samples; preparing samples for laboratory analysis and delivering them to the appropriate institutions; compiling and interpreting early data; and reporting preliminary findings at scientific conferences. We visited 43 wells, 14 springs, about 10 surface flow sites, and we established a precipitation collector in the basin. Well over 100 samples have been analyzed for stable isotopes and major anions, including enough samples to produce respectable time series for the surface and precipitation sites. Preliminary analyses of samples where tritium was expected in detectable quantities reveal travel times of 1-10 years from local recharge sources to spring outlets in the mountains, and rapid MFR travel times on the order of 10-40 years in Ramsey Canyon. Uncorrected early radiocarbon data show apparent ages ranging from modern (the same samples had detectable tritium) in the mountain block to about 18,000 radiocarbon years near gaining reaches of the river. Stable isotopes in groundwater had a gradient with elevation similar to gradients reported for isotopes in regional precipitation. The dominance of winter recharge over monsoon recharge is evident and will be more precisely quantified when external data are considered. Arun Wahi and Matt Baillie authored separate posters presenting different aspects of the data at the 4th Annual SAHRA Meeting. Arun also presented his poster at the 2nd Annual ISPE-Fest Poster Competition (Institute for the Study of Planet Earth), where it received 1st Place in the natural sciences division. Finally, the Kartchner Caverns display was completed and installed in the fall.
Plans for the upcoming year:
For 2006 our focus will be on publication of the results. To date our results have only been published as abstracts and as a MS Thesis. We will search for an appropriate peer-reviewed journal in which to publish.
For 2005 our focus will be on publication of the results. Sample analyses should be complete by early January. We will also attempt analysis of the samples already collected for sulfur isotopes in February. We will search for an appropriate peer-reviewed journal in which to publish findings, and Arun Wahi will defend and submit a master's thesis by the end of the academic year. Well owners will be provided final data obtained from their wells, and we will meet with stakeholders when interest arises, beginning with the Upper San Pedro Partnership at one of their monthly meetings this spring.
Organization Involvement:
Development of Kartchner Caverns display on "Cave Hydrology"
Shared Resources / Joint Activities:
N/A
Location: Wilcox, AZ, USA
URL: N/A
Upper San Pedro Partnership
Organization Involvement:
We arranged to make a presentation to the Technical Committee of the Upper San Pedro Partnership (USPP). This was done for two reasons. First, as the major stakeholder group in the basin we wanted to take the opportunity to present our research plan and gather their advice as to how the project could be improved or modified to meet their management needs as well as our research needs. The second reason was to obtain the permission and contacts needed for well sampling. The presentation was well received and the members of the technical committee thought the research had the possibility of
greatly increasing their understanding of the hydrology of the Huachuca Mountains.
Shared Resources / Joint Activities:
The committee indicated a willingness to fund additional research on this topic if
the initial results generated from TRIF funding proved promising.
Organization Involvement:
In support of outreach efforts associated with this project, we traveled to Kartchner
Caverns in early June to meet with State Park officials and other TRIF funded
researchers to discuss the development of an interactive display on hydrologic
literacy to be housed in the Kartchner Caverns Discovery Center. Researchers
associated with this project agreed to develop content associated with Cave
Hydrology, Mountain Block and Mountain Front Recharge, Isotope Dating of Cave
formations and Groundwater Ages and Basin Hydrology (linking the cave and
mountains to the San Pedro River).
Shared Resources / Joint Activities:
Exhibit space at the Kartchner Caverns Discovery Center.
Organization Involvement:
We met with USGS researchers working in the San Pedro basin. This was done to discuss how our results might assist current USGS activities and to garner advice on well sampling locations. In particular we focused on how our age dating information might be used to calibrate the latest USGS groundwater model for the basin.
