Solute Flux in Paired Watersheds, Valles Caldera National Preserve, NM: Biology or Hydrology- A Catchment Comparison of Nutrient Dynamics J.M. Kostrzewski and P.D. Brooks Department of Hydrology and Water Resources, University of Arizona Sustainability of Semi-Arid Hydrology and Riparian Areas NSF-STC Introduction TS-B24 La Jara Creek Spatial and temporal patterns in water chemistry are related to both catchment interactions between vegetation, soils, and subsurface parent materials and to instream processing of biogeochemical solutes. Significant progress on quantifying how these systems control nutrient concentrations has been ongoing in both catchment-scale hydrological research and in ecologically-based instream studies.  These discipline specific results have led to recent investigations of hydrologic flowpaths to isolate biogeochemical hot spots and hot moments, allowing a definitive assessment of biogeochemical cycling and export on a catchment scale. We evaluated how variable water sources and flowpaths influence the carbon and nutrient content of surface waters in the Valles Caldera National Preserve (VCNP) in northern New Mexico. Our goal was to answer: How do temporal variations in catchment hydrological processes affect instream carbon and nutrient patterns? How are carbon and nutrient concentrations modified during transport? Redondo Creek Table 1: Redondo and La Jara Creek mean nutrient concentrations (1σ) separated by hydrologically defined periods. Figure 2b: La Jara Creek stream conservative solute ratios are clustered near ground water, however the spread of the group is seasonal in nature. Figure 2a: Redondo Creek stream conservative solute ratios are clustered in two groups winter/snowmelt and summer baseflow/monsoon. Regional geothermal data were obtained from other studiesc,d,e. Variation in La Jara stream chemistry is related to seasonal patterns in hydrology. During the snowmelt season (shaded blue), infiltrating melt water displaces a mixture of soil and groundwater into the stream; for the following few months streamflow is a mixture of soil water, groundwater, and snowmelt; during monsoon (shaded green), the stream water is a mixture of only rain and ground water. ‘Older’ groundwater (GW) dominated winter waters (shaded yellow) are pushed into the stream by snowmelt (shaded blue) until input of lower concentrations of ‘new’ waters compose summer baseflow and monsoon (shaded green). Figure 1: The Valles Caldera National Preserve (106º33’23”W, 35º52’19”N) is located in northern New Mexico in the Jemez Mountains. The two research catchments, Redondo Creek (green) and La Jara Creek (purple). We selected two montane watersheds with similar vegetation and elevations, but with differing geology and source waters. La Jara Creek (LJC) catchment (3.7 km2) has an elevation range of 2728 to 3414 m, and a vegetative cover of mixed conifer, aspen, and ponderosa pine. Redondo Creek (RC) catchment (13.4 km2) is linked to a deep geothermal aquifer and has an elevation range of 2485 to 3414 m also with a vegetation transition from mixed conifer to ponderosa pine interspersed with oak scrubland and aspen stands. Figure 3b: Observed/Predicted nutrient ratios are based on conservative solute mixing at La Jara Creek. Values near 1 suggest conservative transport of solutes produced in the catchment, < 1 are indicative of consumption during transport, > 1 are representative of production. Figure 3a: Observed/Predicted nutrient ratios are based on conservative solute mixing at Redondo Creek. Values near 1 suggest conservative transport of solutes produced in the catchment, < 1 are indicative of consumption during transport, > 1 are representative of production. RC and LJC have two flush events that result in high carbon and nutrient concentrations (Table 1) from terrestrial sources in streamwater: the more pronounced snowmelt flush (late April) and the monsoon flush (July). Following snowmelt, measured nitrogen concentrations are lower than predicted in RC suggesting an N uptake during transport possibly due to longer flowpaths, and hence longer time for biological mediation in transport.  In contrast, hydrologic residence times in LJ are shorter and possibly limit opportunities for N uptake during transit. Early season phosphate concentrations were lower than predicted in both catchments.  The concentrations increase at the same time as the soil end member becomes influential in both catchments.  This suggests an early season sorption in the subsurface and a flushing/desorption later in the season. Observed DOC observed/predicted ratios in both catchments were scattered at or below the line of prediction suggesting consumption/removal during transport outweighs production throughout the summer. Methods We sampled the outlet of each catchment at bi-weekly to daily intervals from February through August 2005, and installed rhizon suction lysimeters in the organic soil layer to collect soil solutions on a monthly basis. We analyzed for conservative solutes, dissolved carbon and nutrient species, and water isotopes (stream samples only). Source water contributions were calculated using a 3 end member conservative solute mixing modela: [Cl-] f1 + [Cl-] f2 +[Cl-] f3 = [Cl-]stream (1) [SO42-] f1 + [SO42-] f2 +[SO42-] f3 = [SO42-]stream (2) f1 + f2 + f3 = 1 (3) Stream values outside of the mixing triangle were estimated using a projection methodb. Using these source water mixing ratios, we were able to make hydrology-based predictions of instream nutrient concentrations (DOC, DON, Nitrate+Nitrite, PO4-P) . Predicted concentrations were then compared to observed instream nutrient concentrations to quantify biogeochemical processing. Summary Seasonal changes in conservative solute ratios indicate differences in hydrologic flowpaths in these neighboring catchments. RC: Conservative solutes indicate two groundwater sources (older, thermal and newer hillslope/spring), that mix with precipitation. We see an influence of RC soil solutes in the early summer and monsoon. LJC: Conservative solutes indicate the presence of one ‘newer’ groundwater (springs) source that mixes with precipitation. The LJ soil solutes affect stream chemistry in snowmelt and early summer. Seasonal changes in flowpaths and biological activity influence nutrient concentrations: Winter baseflow: Low hydrologic input creates higher residence times; however, biological activity is lowered due to temperature constraints. Presumably consumption and physical sorbtion are acting to remove the organic matter and phosphate.   Snowmelt: Hydrologic input is high and the catchment is flushed of solutes accumulated since the end of the previous monsoon season. Summer baseflow: Hydrologic throughput is lower and biological activity is high.  DOM is produced and consumed, and nitrate concentrations are reduced due to high demand.   Monsoon: Biological activity remains high, while hydrologic connectivity with terrestrial sources increases.  Production and consumption of DOM continues, and nitrate uptake is high in transport.  Phosphate and DON are flushed in LJ by storm events, but we do not see a response in the well-buffered RC. Spatial and temporal variability in hydrologic flowpaths provide a template for evaluating biologically mediated sources, sinks, and cycling of carbon and nutrients.  Periods of hydrologic connectivity provide transport mechanisms for terrestrial solutes, and alleviate substrate limitation on local biotic communities. A combination of both hydrologic transport and biologic mediation are necessary to fully understand nutrient dynamics in these neighboring catchments. Acknowledgements I would like to thank H. Adams, M.K. Amistadi, D. Brosnihan, L. Klasner, J. Koch, F. Liu, T. Meixner, K. Musselman, and A. Rinehart all who helped in fun field work or tedious lab analyses. I also would like to thank Dr. Robert Parmenter and the VCNP staff for historic and geographic knowledge and logistical aid. Questions and comments can be emailed to Jen Kostrzewski at: jenk@hwr.arizona.edu LITERATURE CITED: cGoff, F. & J.N. Gardner. 1994. Evolution of a mineralized geothermal system, Valles Caldera, NM. Economic Geology 89 (8): 1803-1832. aHooper, R., N. Christophersen, N. Peters. 1990. Modelling streamwater chemistry as a mixture of soilwater end-members — An application to the Panola Mountain catchment, Georgia, U.S.A. J. Hydrology 116(1-4):321-343.  bLiu, FJ, M. William, N. Caine. 2004. Source waters and Flowpaths in an alpine catchment, Colorado Front Range, USA. WRR 40(9):W09401. dRzenca, B. & D. Schulze-Makuch. 2003. Correlation between microbiological and chemical parameters of some hydrothermal springs in New Mexico, USA. J. Hydrology 280 (1-4):272-284. eVuataz F.D., & F. Goff. 1986. Isotope geochemistry in thermal and non-thermal waters of the Valles Caldera, Jemez Mountains, Northern New Mexico. J. Geophysical Research 91(B2): 1835-1853.