The isotopic ratios of nitrate (δ15NNO3 and δ18ONO3) are common tracers of nitrate source and transformation in soil and aquatic systems. However, investigation of their seasonality is limited owing to seldom reported multi-year datasets. This study collects groundwater and springhead samples, measures and analyzes a six-year dataset from the inner bluegrass karst region of central Kentucky, USA to analyze seasonal patterns of nitrate isotopic ratios and investigate their controls. We observe distinct isotopic measurements in groundwater and at the springhead during the wetter winter-spring season (δ15NNO3 = 6.0 ± 1.6‰ and δ18ONO3 = 1.6 ± 2.3‰) compared to the drier fall season (δ15NNO3 = 9.0 ± 1.1‰ and δ18ONO3 = 3.4 ± 1.7‰). Results of time series analyses and empirical mode decomposition suggest the seasonality of the isotopic ratios in the aquifer are controlled by intra-annual variability of soil nitrate isotopic composition, with leaching of nitrate mineralized from soil nitrogen dominating the winter signal and partially denitrified nitrate dominating the fall signal. Results show seasonality of the denitrification line and seasonality of the Rayleigh diagram attributed to the dominant soil nitrogen source and the denitrification controls. Meta-analysis of data from 28 journal papers reporting δ15NNO3 and δ18ONO3 measurements of groundwater and streamwater show a continuum of denitrification-controlled to end-member-controlled results. Groundwater and surface water studies with a dominant end-member of nitrate show potential estimating denitrification rates with assistance from δ15NNO3 and δ18ONO3 as tracers. Studies with water exhibiting relatively fast transport from multiple nitrate sources show potential for δ15NNO3 and δ18ONO3 as tracers in end-member mixing. Researchers should exercise caution when multiple sources exhibit some denitrification influence, as equifinality will be problematic in numerical modelling.

Exposure to tobacco smoke and radon cause lung cancer. Radioactive decay of naturally occurring uranium in bedrock produces radon. Seasonality, bedrock type, age of home, and topography have been associated with indoor radon, but the research is mixed. The study objective was to examine the relationships of geologic (soil radon and bedrock) and seasonal (warm and cold times of the year) factors with indoor home radon values in citizen scientists’ homes over time, controlling for atmospheric conditions, topography, age of home, and home exposure to tobacco smoke. We collected and analyzed indoor radon values, soil radon gas concentrations, and dwelling- and county-level geologic and atmospheric conditions on 66 properties in four rural counties during two seasons: (1) summer 2021 ( n = 53); and (2) winter/spring 2022 ( n = 52). Citizen scientists measured indoor radon using Airthings radon sensors, and outdoor temperature and rainfall. Geologists obtained soil radon measurements using RAD7 instruments at two locations (near the dwelling and farther away) at each dwelling, testing for associations of indoor radon values with soil values, bedrock type, topography, and atmospheric conditions. Bedrock type, near soil radon levels, home age, and barometric pressure were associated with indoor radon. Dwellings built on carbonate bedrock had indoor radon values that were 2.8 pCi/L (103.6 Bq m ^−3 ) higher, on average, compared to homes built on siliclastic rock. Homes with higher near soil radon and those built <40 ago were more likely to have indoor radon ⩾4.0 pCi/L (148 Bq m ^−3 ). With higher atmospheric barometric pressure during testing, observed indoor radon values were lower. Seasonality and topography were not associated with indoor radon level. Understanding relationships among bedrock type, soil radon, and indoor radon exposure allows the development of practical predictive models that may support pre-construction forecasting of indoor radon potential based on geologic factors.

The Ohio River Basin (ORB) is responsible for 35% of total nitrate loading to the Gulf of Mexico yet controls on nitrate timing require investigation. We used a set of submersible ultraviolet nitrate analyzers located at 13 stations across the ORB to examine nitrate loading and seasonality. Observed nitrate concentrations ranged from 0.3 to 2.8 mg L−1 N in the Ohio River's mainstem. The Ohio River experiences a greater than fivefold increase in annual nitrate load from the upper basin to the river's junction with the Mississippi River (74–415 Gg year−1). The nitrate load increase corresponds with the greater drainage area, a 50% increase in average annual nitrate concentration, and a shift in land cover across the drainage area from 5% cropland in the upper basin to 19% cropland at the Ohio River's junction with the Mississippi River. Time‐series decomposition of nitrate concentration and nitrate load showed peaks centered in January and June for 85% of subbasin‐year combinations and nitrate lows in summer and fall. Seasonal patterns of the terrestrial system, including winter dormancy, spring planting, and summer and fall growing‐harvest seasons, are suggested to control nitrate timing in the Ohio River as opposed to controls by river discharge and internal cycling. The dormant season from December to March carries 51% of the ORB's nitrate load, and nitrate delivery is high across all subbasins analyzed, regardless of land cover. This season is characterized by soil nitrate leaching likely from mineralization of soil organic matter and release of legacy nitrogen. Nitrate experiences fast transit to the river owing to the ORB's mature karst geology in the south and tile drainage in the northwest. The planting season from April to June carries 26% of the ORB's nitrate and is a period of fertilizer delivery from upland corn and soybean agriculture to streams. The harvest season from July to November carries 22% of the ORB's nitrate and is a time of nitrate retention on the landscape. We discuss nutrient management in the ORB including fertilizer efficiency, cover crops, and nitrate retention using constructed measures.

Knowledge is lacking for sediment organic matter degradation and its influence on water quality for low gradient agriculturally impacted streams despite their importance for freshwater biogeochemical cycles and ecological restoration. We hypothesized degradation rates vary across sediment type and are a function of the connectivity regimes for low gradient systems. We carried out aerobic incubation experiments to assess oxidation and mineralization-nitrification rates and changes in stable isotopic ratios for sediment and dissolved organic matter and nitrate. Sediment originated from erosion across the watershed's surface shows higher carbon oxidation rates (k = 3.9 x 10-3 d-1) and lower nitrogen mineralization-nitrification rates (k = 7.9 x 10-4 d-1) compared to sediment originated from the creek's streambed that integrates algae and other autotrophic matter (k = 1.6 x 10-3 d-1 and k = 3.4 x 10-3 d-1). Differences are attributed to sediment transport of humified and plant matter during storms of high watershed connectivity and sediment transport of autochthonous sediment for low con-nectivity. Results support our hypothesis and suggest that the sediment connectivity regime of the watershed exhibits control on biogeochemical cycling of the stream network. Cumulatively, sediment degradation rates were one to two orders magnitude higher than previously assumed. Sediment rates reflect aerobic waters and place the organic matter as active and comparable to reported turnover of algae and fine sized leaf litter. Stable isotopic ratios of sediment change marginally for the two sediment types for carbon (epsilon = 0.4 %o and 1.3 %o) and nitrogen (epsilon = 3.5 %o and 1.5 %o). Dissolved organic nitrogen of stream water degraded similarly across all ex-periments (k = 1.8 x 10-2 d-1), and turnover rates were an order of magnitude higher than recent rates reported for lake water. Nitrate concentration in the solute increased by an average 35 % during experiments and the nitrogen stable isotopic ratio of nitrate decreased by over 1 %o showing the potential of sediment and dissolved organic matter degradation to influence nitrate flux and its isotopic signal.

Carbon and nitrogen stable isotopic ratios are increasingly used in sediment fingerprinting studies. However, questions remain regarding tracer conservativeness during sediment transport and other error considerations. We investigate conservativeness processes, including carbon oxidation and nitrogen mineralization, using experiments. We also test how other considerations impact the isotopic ratios including algae accrual into temporary sediment deposits in the river, the physical loss of organic matter via disaggregation, concentration dependent mixing, and time-varying isotopic ratios of sediment sources. Results show all processes and considerations can change isotope abundance, however, significance varied. Carbon oxidation, nitrogen mineralization and upland seasonality of sediment sources did not significantly change isotopic ratios. Algae accrual, concentration dependency mixing, physical loss of organic matter during transport, and seasonality of the in-stream sediment source significantly changed the isotopic ratios for the conditions tested. Fertilization significantly impacted the stable carbon isotopic ratio in one case considered. Results from sediment fingerprinting simulations and testing how well the virtual mixture fits the mass balance equation agreed with significance results for tracer changes, and some uncertainty considerations changed fractional contribution of sources by as much as 50%. A noteworthy recommendation is the mean isotopic ratios of sediment sources should be separated by at least 1‰ to lessen tracer conservativeness concerns in fingerprinting simulation. We recommend concentration dependent mixing becomes the accepted practice when using isotopic ratios, however, we warn against using particle size corrections. We recommend the loss of organic matter during disaggregation be accounted for in fingerprinting estimates. We recommend algae accrual in in-stream sediment deposits should either be accounted for or in-stream sediment should be treated as a time-varying source in sediment fingerprinting simulations. Finally, we recommend both the carbon and nitrogen isotopic ratio should be tested as potential tracers because the two tracers performed similarly when testing how well the virtual mixture fits the mass balance equations.

Nitrogen removal rates can vary with time, space, and external environmental drivers, but are underreported for karst environments. We carried out a multi‐year study of a karst conduit where we: (a) measured inputs and outputs of sediment nitrogen (SN and δ15NSed) and nitrate (NO3− and δ15NNO3); (b) developed, calibrated, and applied a numerical model of nitrogen physics and biogeochemistry; and (c) forecasted the impacts of climate and land use changes on nitrate removal and export. Data results from conduit inputs (SN = 0.43% ± 0.07%, δ15NSed = 5.07‰ ± 1.01‰) and outputs (SN = 0.36% ± 0.09%, δ15NSed = 6.45‰ ± 0.71‰) indicate net‐mineralization of SN and increase of δ15NSed (p < 10−2). However, δ15NSed increase cannot be explained by SN mineralization alone and is instead accompanied by immobilization of isotopically heavier mineral nitrogen (δ15NNO3 = 11.25‰ ± 6.96‰). Modeled SN and δ15NSed sub‐routines provided a boundary condition for DIN simulation and improved NO3− model performance (from NSE = 0.06 to NSE = 0.68). Modeled spatial zones of removal occur in close proximity to conduit entrances, where deposition of labile organic matter promotes a three‐fold increase in denitrification (∼60 mg N m−2 d−1). Modeled temporal periods of removal occur during the dry‐season where longer residence times cause up to 90% removal of NO3− inputs. Projected effects of environmental drivers suggest an increase in denitrification (+14.1%); however, this removal is largely offset by greater nitrate soil leaching (+28.1%) from wetter regional climate. Results suggest that conduits underlying mature karst terrain experience spatiotemporal removal gradients, which are modulated by solute and sediment delivery.

The concentration of lanthanides, Y, and Sc, collectively rare earth elements (REE), and their modes of emplacement in Appalachian coals, with emphasis on Manchester and Fire Clay coals in eastern Kentucky are the focus of this investigation. Those coals have distinct REE concentrations and relationships related to the position of the lithotype within the coal seam, such as the ratio of light to heavy lanthanides (LREE/HREE). In the Manchester coal, the high-S upper lithotypes have lower REE concentrations and different LREE/HREE than the low-S lithotypes. In the Fire Clay coal, the position of the lithotype relative to the REE-enriched tonstein is important, with the coal immediately below the tonstein having significantly higher REE than other lithotypes. Lithotypes in the middle of the coal bench above the tonstein have high LREE/HREE, possibly indicating a hydrothermal influence on the REE emplacement.

Excessive nitrate threatens a wide range of water resources, aquatic habitats, and sensitive infrastructure. Despite this problem, tracing a nutrient from its eventual fate back to its origin remains an elusive challenge due to heterogeneity in how nutrient sources and hydrologic pathways are connected. Typically, this problem is underdetermined (i.e., too many unknowns, not enough equations) and cannot be solved with existing methodologies. The theory of optimal transport allows for the solution of underdetermined systems, and here we construct a novel formulation for its use in water quality modeling. Our objective was to develop an optimal transport modeling framework—coupled to Bayesian source unmixing, loadograph pathway separation, and geospatial connectivity analysis—to apportion nitrate loading from three sources (soil, fertilizer, and manure) across three pathways (quick, intermediate, and slow), resulting in nine possible source‐pathway couplings (soil‐quick, soil‐intermediate, …, manure‐slow). We apply this model to a 30 month elemental (NO3−) and isotopic (δ15N and δ18O) nitrate data set from a karst watershed in Kentucky, USA. Modeling results indicate that—of the nine possible source‐pathway couplings—nearly 60% of nitrate export is facilitated by just three: fertilizer‐quick (16.4%), manure‐intermediate (15.4%), and soil‐slow (27.2%). Further, we reinforce the need to explicitly consider heterogeneity in source‐pathway connectivity as homogeneous assumptions lead to erroneous inferences. The applicability of the model, its input requirements, and transferability to other sites is discussed. Lastly, we simulated two land management scenarios (field buffers and septic repair) and demonstrate how optimal transport can be used to test nutrient reduction strategies.

Nitrate (NO3-) fate estimates in turbulent karst pathways are lacking due, in part, to the difficulty of accessing remote subsurface environments. To address this knowledge and methodological gap, we collected NO3-, (delta N-15(NO3), and delta O-18(NO3) data for 65 consecutive days, during a low-flow period, from within a phreatic conduit and its terminal end-point, a spring used for drinking water. To simulate nitrogen (N) fate within the karst conduit, the authors developed a numerical model of NO3- isotope dynamics. During low-flow, data show an increase in NO3- (from 1.78 to 1.87 mg N L-1; p < 10(-4)) coincident with a decrease in delta N-15(NO3)(from 7.7 to 6.8 parts per thousand; p < 10(-3)) as material flows from within the conduit to the spring. Modeling results indicate that the nitrification of isotopically-lighter ammonium (delta N-15(NH4)) acts as a mechanism for an increase in NO3- that coincides with a decrease in delta N-15(NO3). Further, numerical modeling assists with quantifying isotopic overprinting of nitrification on denitrification (i.e., coincident NO3- production during removal) by constraining the rates of the two processes. Modeled denitrification fluxes within the karst conduit (67.0 +/- 19.0 mg N m(-2) d (-1)) are an order-of-magnitude greater than laminar ground water pathways (1-10 mg N m(-2) d(-1)) and an order-of-magnitude less than surface water systems (100-1000 mg N m(-2) d(-1)). In this way, karst conduits are a unique interface of the processes and gradients that control both surface and ground water end-points. This study shows the efficacy of ambient N stable isotope data to reflect N transformations in subsurface karst and highlights the usefulness of stable isotopes to assist with water quality numerical modeling in karst. Lastly, we provide a rare, if not unique, estimate of N fate in subsurface conduits and provide a counterpoint to the paradigm that karst conduits are conservative source-to-sink conveyors. (C) 2019 Elsevier Ltd. All rights reserved.

Atmospheric rivers and tropical cyclones originate in the tropics and can transport high rainfall amounts to inland temperate regions. The purpose of this study was to investigate the response of nitrate (NO3−) pathways, concentration peaks, and stable isotope (δ15NNO3, δ18ONO3, δ2HH2O, δ18OH2O, and δ13CDIC) measurements to these extreme events. A tropical cyclone and atmospheric river produced the number one and four ranked events in 2017, respectively, at a Kentucky USA watershed characterized by mature karst topography. Hydrologic responses from the two events were different due to rainfall characteristics with the tropical cyclone producing a steeper rising limb of the spring hydrograph and greater runoff generation to the surface stream compared to the atmospheric river. Local minima and maxima of specific conductance, δ2HH2O, δ18OH2O, and δ13CDIC coincided with hydrograph peaks for both events. Minima and maxima of NO3−, δ15NNO3, δ18ONO3, and temperature lagged behind the hydrograph peak for both events, and the values continued to be impacted by diffuse recharge during hydrograph recession. Quick-flow pathways accounted for less than 20% of the total NO3− yield, while intermediate (30%) and slow-flow (50%) pathways composed the remaining load. However, hydrograph separation into quick-, intermediate-, and slow-flow pathways was not able to predict the timing of NO3− concentration peaks. Rather, the intermediate-flow pathway is conceptualized to experience a shift in porosity, associated with a change from epikarst macropores and fissures to soil micropores, with the arrival of water from the latter component likely causing peak NO3− concentration at the spring. Our results suggest that a more discretized conceptual model of pathways may be needed to predict peak nutrient concentration in rivers draining karst topography.

Nitrogen (N) contamination within agricultural-karst landscapes and aquifers is widely reported; however, the complex hydrological pathways of karst make N fate difficult to ascertain. We developed a hydrologic and N numerical model for agricultural-karst, including simulation of soil, epikarst, phreatic, and quick flow pathways as well as biochemical processes such as nitrification, mineralization, and denitrification. We tested the model on four years of nitrate (NO3-) data collected from a phreatic conduit and an overlying surface channel in the Cane Run watershed, Kentucky, USA. Model results indicate that slow to moderate flow pathways (phreatic and epikarst) dominate the N load and account for nearly 90% of downstream NO3- delivery. Further, quick flow pathways dilute NO3- concentrations relative to background aquifer levels. Net denitrification distributed across soil, epikarst, and phreatic water removes approximately 36% of the N inputs to the system at rates comparable to nonkarst systems. Evidence is provided by numerical modeling that NO3- accumulation via evapotranspiration in the soil followed by leaching through the epikarst acts as a control on spring NO3- concentration and loading. Compared to a fluvial-dominated immature karst system, mature-karst systems behave as natural detention basins for NO3-, temporarily delaying NO3- delivery to downstream waters and maintaining elevated NO3- concentrations for days to weeks after hydrologic activity ends. This study shows the efficacy of numerical modeling to elucidate complex pathways, processes, and timing of N in karst systems.

We have previously shown that mice subjected to maternal separation and early weaning (MSEW), a model of early life stress, display exacerbated angiotensin II-dependent obesity-induced hypertension and reduced glomerular filtration rate (GFR) after 16 weeks of high fat diet (HF). In this study, we hypothesized that renin-angiotensin aldosterone system (RAAS) activation in MSEW mice fed a HF for only 12 weeks will precede a decline in GFR and heightened hypertension. MSEW was performed by separating the pups from their mother for 4 to 8 hours from postnatal days (PD) 2 to 16. Mice were weaned at PD 17. Control mice remained undisturbed and were weaned at PD 21. We used 16 MSEW and 14 control litters. Eight-week-old mice were fed on a low fat diet (LF) or HF (10 or 60 % fat Kcal) for 12 weeks. Each litter was represented by one male randomly assigned to each diet. After 11 weeks, 24-hr urine was collected to measure proteins, creatinine, aldosterone and electrolytes by Duo ICAP-OES. At week 12, transcutaneous GFR and blood pressure were measured in all mice. Male MSEW and control mice fed a HF displayed similar blood pressure and metabolic parameters, including water intake (3.4±0.3 vs. 3.4±0.2 ml/day), diuresis (1.1±0.1 vs. 1.0±0.2 ml/day), natriuresis (0.10±0.01 vs. 0.09±0.01 mmol/day), proteinuria (3.5±0.7 vs. 2.9±0.5 mg/day) and GFR (0.97±0.04 vs. 0.96±0.03 ml/min/100g BW). However, MSEW increased kaliuresis (0.41±0.07 vs. 0.23±0.02 mmol/day, p<0.05) and urinary aldosterone (25±4 vs. 11±1 ng/g crea, p<0.05). In addition, MSEW mice displayed increased plasma aldosterone (158±32 vs. 87±17 pg/ml, p=0.056) and lower plasma renin concentration compared to controls (2.3±0.4 vs. 3.7±0.4 ng/mL/30’, p<0.05), suggesting elevated circulating angiotensin II. Only MSEW mice showed increased adiposity (36±1 vs. 24±4, %) and a positive correlation between urinary aldosterone and fat mass (2.6±0.9 vs. -0.1± 0.1, p<0.05). These data indicate that MSEW-induced RAAS overactivation precedes the increases in blood pressure and decline in GFR found in HF-fed mice later in life. Thus, increased angiotensin II, most likely due to elevated fat angiotensinogen, may stimulate adipocyte and/or adrenal-derived aldosterone production exacerbating obesity-hypertension in MSEW mice.

Leaching of solutes below the root zone has been identified as a main source of potential groundwater pollution. The occurrence of preferential flow paths in structured soils can enhance rapid leaching of solutes below the root zone. There is evidence that the actual land use can affect solute displacement by altering soil structure and the abundance of preferential flow paths. In the present study, a field experiment was conducted to assess the impacts of land use (grassland vs. no-till cropland) on profile-scale displacement of bromine (Br) and Brilliant Blue FCF. The objectives were (i) to study both solutes displacement patterns, (ii) to analyze the spatial variation and anisotropic variance structures of the solutes and controlling physical soil properties, and (iii) to analyze soil structure development as a result of the land use system and possible implications for solute displacement. Two ponding infiltration experiments with Potassiumbromide (KBr) and Brilliant Blue FCF were performed on a silt loam soil in Lexington, KY. A total of 30mm multi-tracer solution was infiltrated on an area of 1.20×0.70m. Eleven vertical profile sections (width: 1.10m, depth: 0.80m) were excavated in steps of 0.05m and sampled. Dye stained areas were mapped based on digital image analysis. Small soil samples were taken for Br concentrations, soil texture, and volumetric soil water content at regular intervals along a vertical 0.10×0.10m raster. Vane shear resistance was measured as a proxy for mechanical soil strength. X-ray fluorescence analysis was used to determine total Br contents and the relative SiO2 signal intensity, the latter being used as proxy for soil particle size distribution. Although both experimental sites were under the same land use until some 10 years ago before the current land uses were established, solutes displacement differed between both land uses. The dye-stained patterns revealed a high proportion of non-equilibrium flow through vertically orientated macropores and a less permeable soil matrix at the grassland site. Continuous biological activity since transversion into grassland resulted in these macropores and the absence of any compaction in the subsoil. Large proportions of Br also infiltrated directly into the loose, densely rooted soil matrix close to the surface. Soil structure development at the no-till cropland site was mainly controlled by agricultural operations. The homogeneous Br distribution in the topsoil reflected a less dense soil matrix with a network of well-connected inter-aggregate pores. A residual plough pan restricted solute displacement to deeper soil layers or to groundwater bodies. Although ponding infiltration was applied in this study, the leaching risk for both applied solutes – Br and Brilliant Blue FCF – was rather small.