Rosemary W.H. Carroll

Prairie Wetland Complexes as Landscape Functional Units in a Changing Climate

The wetland complex is the functional ecological unit of the prairie pothole region (PPR) of central North America. Diverse complexes of wetlands contribute high spatial and temporal environmental heterogeneity, productivity, and biodiversity to these glaciated prairie landscapes. Climatewarming simulations using the new model WETLANDSCAPE (WLS) project major reductions in water volume, shortening of hydroperiods, and less-dynamic vegetation for prairie wetland complexes. The WLS model portrays the future PPR as a much less resilient ecosystem: The western PPR will be too dry and the eastern PPR will have too few functional wetlands and nesting habitat to support historic levels of waterfowl and other wetland-dependent species. Maintaining ecosystem goods and services at current levels in a warmer climate will be a major challenge for the conservation community.

Uncertainty analysis of the Carson River mercury transport model

Abstract Three computer models developed by the US Environmental Protection Agency (RIVMOD, WASP5 and MERC4) are used to simulate mercury transport and fate within the Carson River in west-central Nevada. While the transport of inorganic mercury is well understood and model prediction occurs with reasonable accuracy, it is recognized that the processes affecting methylmercury (MeHg) fate are more complicated. During January 1997 an extreme flood occurred which caused significant geomorphic change to the system. Following this flood, model error concerning MeHg water column concentrations increased substantially. This study addresses uncertainty in predicting water column MeHg in the hope of delineating change to MeHg transport caused by the 1997 flood. A preliminary sensitivity analysis shows that both the diffusion rate and the methylation/demethylation ratio ( M / D ) significantly affect MeHg water column concentrations, particularly during low discharge. It is discovered that the measured upper limit of M / D must be constrained from 0.26 to 0.065 to ensure accurate model calibration. A more comprehensive Monte Carlo analysis is conducted to test error propagation associated with uncertainty in methylation and demethylation rate coefficients. Methylation and demethylation rate assignments are assumed to vary independently with each parameter being characterized by a uniform distribution thereby reflecting user decision uncertainty. Monte Carlo results show that model uncertainty, defined by the 90% confidence interval (CI), in the lower portions of the Carson River converges upon 1.9 and 11.6 ng/l for steady discharges of 28.3 m 3 /s (1000 ft 3 /s) and 1.42 m 3 /s (50 ft 3 /s), respectively. The 90% CI encompasses the scatter of the observed data points, proving that it is not possible to suggest, with a high degree of statistical certainty, a significant change has occurred to MeHg transport and fate as a result of the 1997 flood.

Snowmelt controls on concentration-discharge relationships and the balance of oxidative and acid-base weathering fluxes in an alpine catchment, East River, Colorado

Although important for riverine solute and nutrient fluxes, the connections between biogeochemical processes and subsurface hydrology remain poorly characterized. We investigate these couplings in the East River, CO, a high-elevation shale-dominated catchment in the Rocky Mountains, using concentration-discharge (C-Q) relationships for major cations, anions, and organic carbon. Dissolved organic carbon (DOC) displays a positive C-Q relationship with clockwise hysteresis, indicating mobilization and depletion of DOC in the upper soil horizons and emphasizing the importance of shallow flowpaths during snowmelt. Cation and anion concentrations demonstrate that carbonate weathering, which dominates solute fluxes, is promoted by both sulfuric acid derived from pyrite oxidation in the shale bedrock and carbonic acid derived from subsurface respiration. Sulfuric acid weathering dominates during baseflow conditions when waters infiltrate below the inferred pyrite oxidation front, whereas carbonic acid weathering plays a dominant role during snowmelt as a result of shallow flowpaths. Differential C-Q relationships between solutes suggest that infiltrating waters approach calcite saturation before reaching the pyrite oxidation front, after which sulfuric acid reduces carbonate alkalinity. This reduction in alkalinity results in CO2 outgassing when waters equilibrate to surface conditions, and reduces the riverine export of carbon and alkalinity by roughly 33% annually. Future changes in snowmelt dynamics that control the balance of carbonic and sulfuric acid weathering may substantially alter carbon cycling in the East River. Ultimately, we demonstrate that differential C-Q relationships between major solutes can provide unique insights into the complex subsurface flow and biogeochemical dynamics that operate at catchment scales. This article is protected by copyright. All rights reserved.

Evaluating mountain meadow groundwater response to Pinyon-Juniper and temperature in a Great Basin watershed†

This research highlights development and application of an integrated hydrologic model (GSFLOW) to a semiarid, snow-dominated watershed in the Great Basin to evaluate Pinyon-Juniper (PJ) and temperature controls on mountain meadow shallow groundwater. The work used Google Earth Engine Landsat satellite and gridded climate archives for model evaluation. Model simulations across three decades indicated that the watershed operates on a threshold response to precipitation (P) > 400 mm y-1 to produce a positive yield (P-ET; 9%) resulting in stream discharge and a rebound in meadow groundwater levels during these wetter years. Observed and simulated meadow groundwater response to large P correlates with above average predicted soil moisture and with a normalized difference vegetation index (NDVI) threshold value > 0.3. A return to assumed pre-expansion PJ conditions or an increase in temperature to mid-21st century shifts yielded by only ±1% during the multi-decade simulation period; but changes of approximately ±4% occurred during wet years. Changes in annual yield were largely dampened by the spatial and temporal redistribution of evapotranspiration (ET) across the watershed. Yet, the influence of this redistribution and vegetation structural controls on snowmelt altered recharge to control water table depth in the meadow. Even a small-scale removal of PJ (0.5 km2) proximal to the meadow will promote a stable, shallow groundwater system resilient to droughts, while modest increases in temperature will produce a meadow susceptible to declining water levels and a community structure likely to move toward dry and degraded conditions. This article is protected by copyright. All rights reserved.

Sensitivity of Solute Advective Travel Time to Porosities of Hydrogeologic Units

An integral approach is proposed to quantify uncertainty and sensitivity of advective travel time to the effective porosities of hydrogeologic units (HGUs) along groundwater flow paths. The approach is applicable in situations where a groundwater flow model exists, but a full solute transport model is not available. The approach can be used to: (1) determine HGUs whose porosities are influential to the solute advective travel time; and (2) apportion uncertainties of solute advective travel times to the uncertainty contributions from individual HGU porosities. A simple one-dimensional steady-state flow example is used to illustrate the approach. Advective travel times of solutes are obtained based on the one-dimensional steady-state flow results in conjunction with the HGU porosities. The approach can be easily applicable to more complex multi-dimensional cases where advective solute travel time can be calculated based on simulated flow results from groundwater flow models. This approach is particularly valuable for optimizing limited resources when designing field characterization programs for uncertainty reduction by identifying HGUs that contribute most to the estimation uncertainty of advective travel times of solutes.

Factors controlling seasonal groundwater and solute flux from snow-dominated basins

Desert Research Institute, Reno, NV, USA Rocky Mountain Biological Laboratory, Crested Butte, CO, USA Lawrence Berkeley National Laboratory, Berkeley, CA, USA Correspondence Rosemary W. H. Carroll, Division of Hydrologic Sciences, Desert Research Institute, 2215 Raggio Parkway, Reno, NV 89512, USA. Email: rosemary.carroll@dri.edu Funding information Lawrence Berkeley National Laboratory (LBNL), Grant/Award Number: DE‐AC02‐ 05CH11231; U.S. Geological Survey (USGS), Grant/Award Number: G16AP00196

Occurrence, distribution, and seasonality of emerging contaminants in urban watersheds

The widespread occurrence of natural and synthetic organic chemicals in surface waters can cause ecological risks and human health concerns. This study measured a suite of contaminants of emerging concern (CECs) in water samples collected by the U.S. Environmental Protection Agency Region 8 around the Denver, Colorado, metropolitan area. The results showed that 109 of 144 analyzed pharmaceutical compounds, 42 of 55 analyzed waste-indicator compounds (e.g., flame retardants, hormones, and personal care products), and 39 of 72 analyzed pesticides were detected in the water samples collected monthly between April and November in both 2014 and 2015. Pharmaceutical compounds were most abundant in the surface waters and their median concentrations were measured up to a few hundred nanograms per liter. The CEC concentrations varied depending on sampling locations and seasons. The primary source of CECs was speculated to be wastewater effluent. The CEC concentrations were correlated to streamflow volume and showed significant seasonal effects. The CECs were less persistent during spring runoff season compared with baseflow season at most sampling sites. These results are useful for providing baseline data for surface CEC monitoring and assessing the environmental risks and potential human exposure to CECs.

Numerical Simulation of Groundwater Withdrawal within the Mercury Valley Administrative Groundwater Basin, Nevada

A detailed, transient, three-dimensional, finite-difference groundwater flow model was created for the Mercury Valley Administrative Groundwater Basin (MVB). The MVB is a distinct groundwater basin as defined by the State of Nevada and is located partially within the boundary of the Nevada Test Site. This basin is being studied as a potential location for new industrial facilities and therefore would be subject to Nevada water-use limitations. The MVB model was used to estimate the volume of water that could be withdrawn from Mercury Valley without inducing laterally or vertically extensive water-table effects. In each model simulation, water-table drawdown was limited to a maximum of 0.5 m at the boundary of the basin and held within the screened interval of the well. Water withdrawal from Nevada groundwater basins is also limited to the State-defined perennial yield for that area. The perennial yield for the MVB is 27,036 m{sup 3}/day. The one existing water-supply well in Mercury Valley is capable of sustaining significantly higher withdrawal rates than it currently produces. Simulations showed this single well could produce 50 percent of the basin?s perennial yield with limited water-table drawdown. Pumping from six hypothetical water-supply wells was also simulated. Each hypothetical well was placed in an area of high hydraulic conductivity and far from the basins boundaries. Each of these wells was capable of producing at least 50 percent of the basins perennial yield. One of the hypothetical wells could simulate 100 percent of the perennial yield while staying within drawdown limitations. Multi-well simulations where two or more water-supply wells were simultaneously pumping were also conducted. These simulations almost always resulted in very limited lateral and vertical drawdown and produced 100 percent of Mercury Valleys perennial yield. A water-budget analysis was also conducted for each of the various stress simulations. Each of the stress scenarios was compared to a baseline scenario where existing water-supply wells in the model domain were pumped at 2003-2004 average pumping rates. Water-budget analyses showed increased flow from the constant-head boundaries on the north, east, and west sides of the model. Flow to the southern, head-dependent boundary and to springs in the Ash Meadows area remained unchanged.

Numerical Simulation of Groundwater Withdrawal at the Nevada Test Site

Alternative uses of the Nevada Test Site (NTS) may require large amounts of water to construct and/or operate. The only abundant source of water at the NTS is groundwater. This report describes preliminary modeling to quantify the amount of groundwater available for development from three hydrographic areas at the NTS. Modeling was conducted with a three-dimensional transient numerical groundwater flow model.