Science of the Total Environment

Recently, the subsoils of ephemeral stream(arroyos) floodplains in the northern Chihuahuan Desert were discovered to contain large naturally occurring NO3− reservoirs (floodplain:~38,000 kg NO3-N/ha; background:~60 kg NO3-N/ha). These reservoirs may be mobilized through land use change or natural stream channel migration which makes differentiating between anthropogenic and natural groundwaterNO3−sources challenging. In this study, the fate and sources of NO3− were investigated in an area with multiple NO3− sources such as accidental sewer line releases and sewage lagoons aswell as natural reservoirs of subsoil NO3−. To differentiate sources, this study used a large suite of geochemical tools including δ15N[NO3], δ18O[NO3], δ15N[N2], δ13C[DIC], 14C, tritium (3H), dissolved gas concentrations, major ion chemistry, and contaminants of emerging concern (CEC) including artificial sweeteners. NO3− at sites with the highest concentrations (25 to 229 mg/L NO3-N) were determined to be largely sourced from naturally occurring subsoil NO3− based on δ15N[NO3] (\u3c8 ‰) and mass ratios of Cl−/Br− (〈100) and NO3−/Cl− (\u3e1.5). Anthropogenic NO3− was deciphered using mass ratios of Cl−/Br− (\u3e120) and NO3−/Cl− (\u3c1), δ15N[NO3] (\u3e8‰), and CEC detections. Nitrogen isotope analyses indicated that denitrification is fairly limited in the field area. CEC were detected at 67 % of sites including 3H dead sites (\u3c1 pCi/L) with low percent modern carbon-14 (PMC; \u3c30 %). Local supply wells are 3H dead with low PMC; as 3H does not re-equilibrate and 14C is very slow to re-equilibrate during recirculation through infrastructure, sites with low PMC, 3H \u3c 1 pCi/L, and CEC detections were interpreted as locations with substantial anthropogenic groundwater recharge. Neotame was used to identify locations of very recent (\u3c15 years before present) or ongoing wastewater influxes to the aquifer. This work shows the important influence of naturally occurring subsoil NO3− reservoirs on groundwater in arid regions and the major contribution of artificial recharge

Geochemical evolution of uraniferous soda lakes in Eastern Mongolia

Extremely high concentrations of U were discovered in hypersaline soda lakes in eastern Mongolia. The lakes are small, shallow (<1km², <1m), and terminal with variable salinity. The origin and fate of U in these lakes was investigated using geochemical analyses and modeling, using samples collected from lakes and lake pore waters, wells and a stream. Samples were analyzed for Sr and U isotopes, cations, trace metals, anions, total inorganic carbon (TIC), and unstable field parameters. A representative groundwater in the field area is dilute and alkaline, with pH=7.9, 10 mmol L⁻¹ of TIC and 5 mmol L⁻¹ Cl⁻. In contrast, a representative lake water is pH(similar to)10 with TIC and Cl⁻ each more than 1000 mmol L⁻¹. Uranium concentrations in lake waters range from 0.24 to more than 62.5 (Greek small letter mu)mol L⁻¹, possibly making these lakes the highest naturally occurring U concentrations ever reported in a natural water. Groundwater concentrations of U range from 0.03 and 0.43 (Greek small letter mu)mol L⁻¹. The U is natural and derived from groundwater discharging to stable closed basin lakes. Waters are concentrated by evaporation and U(VI) is chelated by CO₃⁻² to form the highly soluble UO₂(CO₃)₃⁻⁴. Two sets of well waters with corresponding lake discharge waters were analyzed for U isotopes. Unnatural concentrations of ²³⁵U were tested for, the presence of which would indicate fallout-derived U. The average of four samples was ²³⁸U/²³⁵U=136(plus or minus)2 indicating that the U is naturally derived. (Greek small letter delta)²³⁴U between one well and its corresponding discharge lake had similar (Greek small letter delta)²³⁴U values ([Greek small letter delta]²³⁴U=837.6- 858.5). The other sample pair however, revealed significant differences between the well and its discharge lake ([Greek small letter delta] ²³⁴U=303.6-1530). This suggests nuclide recoil is significantly enriching one of the lakes with ²³⁴U beyond secular equilibrium during alpha-decay of ²³⁸U in lake sediments or along the groundwater flow path. Modeled evaporation of lakes demonstrates that a U-mineral phase is likely to precipitate during evaporation. Strontium isotopes varied in groundwaters between ⁸⁷Sr/⁸⁶Sr= 0.706192-0.709776 and in lakes ⁸⁷Sr/⁸⁶Sr=0.708702-0.709432. Cretaceous mafic rocks likely account for low ⁸⁷Sr/⁸⁶Sr values while Cretaceous alkaline rhyolites account for the high ⁸⁷Sr/⁸⁶Sr values. High concentrations of U, Na, Cl⁻, and K correlate to radiogenic Sr in lake waters indicating that U is sourced from local Cretaceous alkaline rhyolites.Geological Science

Seasonal and interannual variability in the hydrology and geochemistry of an outlet glacier of the Greenland Ice Sheet

Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution February 2016In the spring and summer within the ablation zone of the Greenland Ice Sheet (GrIS), meltwater drains to the ice sheet bed through an evolving network of efficient channelized and inefficient distributed drainage systems. Distributed system drainage is a key component in stabilizing GrIS velocity on interannual time scales and controlling geochemical fluxes. During the spring and summer of 2011 and 2012, I conducted fieldwork at a large outlet glacier in southwest Greenland underlain by metamorphic silicate rocks. Data collected from a continuous 222Rn monitor in the proglacial river were used as a component of a mass balance model. I demonstrated that Jdis, the 222Rn fraction derived from the distributed system, was >90% of the 222Rn flux on average, and therefore, 222Rn can be used as a passive flow tracer of distributed system drainage. Supraglacial meltwater runoff estimated using two independent models was compared with ice velocity measurements across the glacier’s catchment. Major spikes of Jdis occurred after rapid supraglacial meltwater runoff inputs and during the expansion of the subglacial channelized system. While increases in meltwater runoff induced ice acceleration, they also resulted in the formation of efficient subglacial channels and increased drainage from the distributed system, mechanisms known to cause slower late summer to winter velocities. Sr, U, and Ra isotopes and major and trace element chemistry were used to investigate the impact of glacial hydrology on subglacial weathering. Analysis of partial and total digestions of the riverine suspended load (SSL) found that trace carbonates within the silicate watershed largely controlled the 87Sr/86Sr ratio in the dissolved load. Experiments and sampling transects downstream from the GrIS demonstrated that δ234U in the dissolved phase decreased with increasing interaction with the SSL. The (228Ra/226Ra) value of the dissolved load was significantly higher than that of the SSL and therefore, was not the result of the source rock material but of extensive mineral surface weathering and the faster ingrowth rate of 228Ra (t1/2=5.75 y) relative to 226Ra (t1/2=1600 y). In summary, extensive, repeated cycles of rapid supraglacial meltwater runoff to subglacial drainage networks leads to increased distributed system drainage and mineral weathering.Funding for this work was provided by the U.S. National Science Foundation Arctic Natural Sciences Program (ANS-1256669); Woods Hole Oceanographic Institution Arctic Research Initiative, Ocean Ventures Fund, and Ocean Climate Change Institute; United Kingdom Natural Environment Research Council studentship (NE/152830X/1); the Carnegie Trust, Edinburgh University Development Trust

Rapid development and persistence of efficient subglacial drainage under 900 m-thick ice in Greenland

Intensive study of the Greenland Ice Sheet's (GrIS) subglacial drainage has been motivated by its importance for ice dynamics and for nutrient/sediment export to coastal ecosystems. This has revealed consistent seasonal development of efficient subglacial drainage in the lower ablation area. While some hydrological models show qualitative agreement with field data, conflicting evidence (both field- and model-based) maintains uncertainty in the extent and rate of efficient drainage development under thick (∼1 km) ice. Here, we present the first simultaneous time series of directly-observed subglacial drainage evolution, supraglacial hydrology and ice dynamics over 11 weeks in a large GrIS catchment. We demonstrate development of a fast/efficient subglacial drainage system extending from the margin to beneath ice >900 m thick, which then persisted with little response to highly variable moulin inputs including extreme melt events and extended periods (2 weeks) of low melt input. This efficient system evolved within ∼3 weeks at a moulin initiated when a fracture intersected a supraglacial river (rather than hydrofracture and lake drainage). Ice flow response to surface melt inputs at this site follows a pattern commonly observed in the lower GrIS ablation area, and by assuming a strong relationship between ice dynamics and subglacial hydrology, we infer that efficient subglacial drainage evolution is widespread under 900 m-thick ice in west Greenland. This time series of tracer transit characteristics through a developing and then persistent efficient drainage system provides a unique data set with which to validate and constrain existing numerical drainage system models, extending their capability for simulating drainage system evolution under current and future conditionspublishedVersio

Large subglacial source of mercury from the southwestern margin of the Greenland Ice Sheet

The Greenland Ice Sheet is currently not accounted for in Arctic mercury budgets, despite large and increasing annual runoff to the ocean and the socio-economic concerns of high mercury levels in Arctic organisms. Here we present concentrations of mercury in meltwaters from three glacial catchments on the southwestern margin of the Greenland Ice Sheet and evaluate the export of mercury to downstream fjords based on samples collected during summer ablation seasons. We show that concentrations of dissolved mercury are among the highest recorded in natural waters and mercury yields from these glacial catchments (521–3,300 mmol km−2 year−1) are two orders of magnitude higher than from Arctic rivers (4–20 mmol km−2 year−1). Fluxes of dissolved mercury from the southwestern region of Greenland are estimated to be globally significant (15.4–212 kmol year−1), accounting for about 10% of the estimated global riverine flux, and include export of bioaccumulating methylmercury (0.31–1.97 kmol year−1). High dissolved mercury concentrations (~20 pM inorganic mercury and ~2 pM methylmercury) were found to persist across salinity gradients of fjords. Mean particulate mercury concentrations were among the highest recorded in the literature (~51,000 pM), and dissolved mercury concentrations in runoff exceed reported surface snow and ice values. These results suggest a geological source of mercury at the ice sheet bed. The high concentrations of mercury and its large export to the downstream fjords have important implications for Arctic ecosystems, highlighting an urgent need to better understand mercury dynamics in ice sheet runoff under global warming

Utility of 222Rn as a passive tracer of subglacial distributed system drainage

This paper is not subject to U.S. copyright. The definitive version was published in Earth and Planetary Science Letters 462 (2017): 180-188, doi:10.1016/j.epsl.2016.12.039.Water flow beneath the Greenland Ice Sheet (GrIS) has been shown to include slow-inefficient (distributed) and fast-efficient (channelized) drainage systems, in response to meltwater delivery to the bed via both moulins and surface lake drainage. This partitioning between channelized and distributed drainage systems is difficult to quantify yet it plays an important role in bulk meltwater chemistry and glacial velocity, and thus subglacial erosion. Radon-222, which is continuously produced via the decay of 226Ra, accumulates in meltwater that has interacted with rock and sediment. Hence, elevated concentrations of 222Rn should be indicative of meltwater that has flowed through a distributed drainage system network. In the spring and summer of 2011 and 2012, we made hourly 222Rn measurements in the proglacial river of a large outlet glacier of the GrIS (Leverett Glacier, SW Greenland). Radon-222 activities were highest in the early melt season (10–15 dpm L−1), decreasing by a factor of 2–5 (3–5 dpm L−1) following the onset of widespread surface melt. Using a 222Rn mass balance model, we estimate that, on average, greater than 90% of the river 222Rn was sourced from distributed system meltwater. The distributed system 222Rn flux varied on diurnal, weekly, and seasonal time scales with highest fluxes generally occurring on the falling limb of the hydrograph and during expansion of the channelized drainage system. Using laboratory based estimates of distributed system 222Rn, the distributed system water flux generally ranged between 1–5% of the total proglacial river discharge for both seasons. This study provides a promising new method for hydrograph separation in glacial watersheds and for estimating the timing and magnitude of distributed system fluxes expelled at ice sheet margins.U.S. National Science Foundation Arctic Natural Sciences Program (ANS-1256669); Woods Hole Oceanographic Institution Arctic Research Initiative, Ocean Ventures Fund, and Ocean Climate Change Institute; United Kingdom Natural Environment Research Council studentship (NE/152830X/1); the Carnegie Trust, Edinburgh University Development Trust

Uranium isotope composition of waters from South Texas uranium ore deposits

Redox conditions and associated changes in mobility of uranium (U) are tightly linked to a multitude of challenges connected with U mining in sandstone-hosted deposits and new methods that directly measure reduction or oxidation of U can inform on these questions. A novel proxy for understanding U redox chemistry has recently emerged, the volume dependent isotopic fractionation of uranium-238 (²³⁸U) from uranium-235 (²³⁵U). Novel measurements of ²³⁸U/²³⁵U isotopic ratio are combined with measurements of the more commonly utilized uranium-234/uranium-238 activity [(²³⁴U/²³⁸U)] ratio, as both isotopic ratios can be measured simultaneously. However, application of both U isotopic ratios in the contexts of exploration and environmental remediation of U ores requires characterization of these isotopic ratios across a variety of redox settings. Here, ²³⁸U/²³⁵U and (²³⁴U/²³⁸U) ratios are examined from eight transects in two U ore bodies (the Rosita and Kingsville Dome deposits) in South Texas; these sites are classic roll front deposits and exhibit a wide variety of both natural and altered redox conditions. Across all transects it is observed that (²³⁴U/²³⁸U) ratios decrease systematically towards the ore body from both the oxidizing and reducing sides, irrespective of whether the site has been mined or not. This pattern reflects geologically recent and significant U leaching and mobility and is characteristic of active roll fronts. Overall δ²³⁸U values in these transects decrease systematically towards the reducing zone. A simple Rayleigh fractionation model, where U ore is deposited from an increasingly isotopically depleted reservoir of dissolved U best explains the overall trend; very negative δ²³⁸U values likely reflect multiple cycles of U deposition and dissolution. The South Texas data set indicates that both (²³⁴U/²³⁸U) ratios and δ²³⁸U values can be variable at an individual mine site. However, overall low (²³⁴U/²³⁸U) ratios and negative δ²³⁸U values are characteristic of active roll front deposits. The comprehensive U isotopic composition of both ores and well waters represents a powerful new tool in prospecting of sandstone-hosted U ore and in environmental remediation following extraction of U ore

Westwards component of ice surface velocity on the Greenland Ice Sheet measured in spring/summer 2012

Ice surface motion was recorded by five dual-frequency Leica SR520 GPS receivers deployed on poles drilled 2 m into the ice surface, within 700 m of Moulin L41A at 66.97N -49.27E. GPS data were post-processed kinematically (King, 2004, http://dx.doi.org/10.3189/172756504781829747) with Track v.1.27 software (Chen, 1998, Ph.D. thesis, Cambridge MA, USA) against bedrock-mounted reference stations using a precise ephemeris from the International GNSS Service )Dow et al., 2009, http://dx.doi.org/10.1007/s00190-008-0300-3). Reference stations were located 1 km from the terminus of Russell Glacier and at Kellyville, giving baseline lengths less than 41 km. Due to gaps in the time series caused by power outage, we averaged the horizontal velocities recorded at the five stations with the fewest gaps to give a single record. Positions were recorded at 30 s intervals; 1-hr means were then smoothed using a 5-point binomial filter. Since there was generally little difference in velocity between the stakes, the mean velocity across the network gives a better indication of the seasonal pattern of ice motion with fewer gaps than in the individual records. Velocities are centred differences of hourly displacements. GPS stakes required periodic re-drilling as they gradually melted out