[1] In recent decades, the Rocky Mountain West has been one of the fastest growing regions in the United States. Headwater streams in mountain environments may be particularly susceptible to nitrogen enrichment from residential and resort development. We utilized stream water chemistry from six synoptic sampling campaigns combined with land use/land cover (LULC) and terrain analysis in geostatistical modeling to examine the spatial and seasonal variability of LULC impacts on stream water nitrate. Stream water nitrate was spatially correlated for longer distances during the dormant season than during the growing season, suggesting the importance of biological retention. Spatial linear models indicated that anthropogenic sources best predicted stream water nitrate in the dormant season, while variables describing biological processing were the best predictors in the growing season. This work demonstrates the importance of (1) incorporating spatial relationships into water quality modeling and (2) investigating stream water chemistry across seasons to gain a more complete understanding of development impacts on stream water quality.

Ground water nitrate concentrations on Nantucket Island, Massachusetts, were analyzed to assess the effects of land use on ground water quality. Exploratory data analysis was applied to historic ground water nitrate concentrations to determine spatial and temporal trends. Maximum likelihood Tobit and logistic regression analyses of explanatory variables that characterize land use within a 1000-foot radius of each well were used to develop predictive equations for nitrate concentration at 69 wells. The results demonstrate that historic nitrate concentrations downgradient from agricultural land are significantly higher than nitrate concentrations elsewhere. Tobit regression results demonstrate that the number of septic tanks and the percentages of forest, undeveloped, and high-density residential land within a 1000-foot radius of a well are reliable predictors of nitrate concentration in ground water. Similarly, logistic regression revealed that the percentages of forest, undeveloped, and low-density residential land are good indicators of ground water nitrate concentration > 2 mg/L. The methodology and results outlined here provide a useful tool for land managers in communities with shallow water tables overlain with highly permeable materials to evaluate potential effects of development on ground water quality.

[1] The link between watershed nitrogen (N) loading and watershed nitrate (NO 3 À ) export is poorly understood yet critical to addressing the growing global problem of watershed N enrichment. We introduce the Big Sky nutrient export model (BiSN) which incorporates spatial stream water chemistry, data from instream tracer additions and geologic weathering experiments, and terrain and land use analysis to quantify the spatial variability of watershed sensitivity to N loading and the relative importance of upland, riparian, and instream N retention (storage, removal, or transformation) across land use/land cover (LULC) and landscape positions. Bayesian Markov chain Monte Carlo (MCMC) methods were used for model specification and were helpful in assessing model and parameter uncertainty and advancing understanding of the primary processes governing watershed NO 3 À export. Modeling results revealed that small amounts of wastewater loading occurring in watershed areas with short travel times to the stream had disproportionately large impacts on watershed nitrate (NO 3 À ) export compared to spatially distributed N loading or localized N loading in watershed areas with longer travel times. In contrast, spatially distributed N inputs of greater magnitude (terrestrial storage release and septic systems) had little influence on NO 3

[1] A 30 day time course laboratory weathering experiment was conducted using rock samples collected from the West Fork of the Gallatin River watershed (WFW) in southwestern Montana, USA. The goal of these experiments was to quantify the amount of labile nitrogen in rock samples collected from the watershed and determine if chemical weathering is a source of dissolved nitrogen in stream water. Several rock samples investigated produced nitrate in significantly higher concentrations than the silica bead control (p < 0.05), and the data were consistent with elevated NO 3 À concentrations measured in associated WFW streams. Isotopic analyses of 15 N-NO 3 in 22 stream water samples from the WFW and four rock:water extracts from the laboratory experiments indicated that the isotopic composition of NO 3 was comparable with rocks and stream water samples in the same watershed and differed strongly from waters downstream of development. We suggest that the NO 3 À measured in WFW streams includes nitrogen derived from mineral dissolution products from soils and rock. The results presented herein further indicate that rock weathering is a source of stream water N in the West Fork watershed and inform water quality assessment, total maximum daily load development, and the relative influences of natural and anthropocentrically derived N sources across this developing mountain watershed.