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The Phosphorus (P) Index risk assessment tool has been widely adopted across the United States to identify and rank site vulnerability to P runoff as part of the Natural Resources Conservation Service nutrient management planning (NMP) process. However, limited success has been achieved in addressing the risk of P loss by subsurface flow pathways, despite its relative importance in certain areas of the United States, particularly in those U.S. states dominated by karst terrain. Here we review how states with varying land areas classified as having karst features address the risk of P runoff during the NMP process. Indices adopted in Illinois and Indiana require setbacks (widths 15–72 m) around surface karst features. The remaining states with karst address the risk of P loss in NMP development rather than the application of a P Index. Given the spatially variable hydrogeologic properties of karst, technically rigorous field‐scale factors are unlikely to be developed in the near future.

Isotopes—atoms of the same chemical element with varying mass and behavior—are used widely in cave research to understand modern and ancient karst systems. Radioisotopes decay spontaneously, and their well-defined decay rates are used for age dating water and materials in caves. Stable isotopes, which do not decay, are used to study modern karst including distinguishing sources of cave air, water, and contaminants and to study past climates and environments through interpretations of isotopes preserved in cave formations (speleothems) and sediments. Radioisotopes used in cave research include carbon-14 in charcoal or bones, uranium-234 and thorium-230 in speleothems, aluminum/beryllium ratios of quartz sediment transported into caves, and tritium and chlorine-36 in karst waters. Stable isotopes used to study caves include oxygen and hydrogen isotopes of water, inorganic carbon (both dissolved and bound in calcite of bedrock or speleothems), and possible contaminants in water, such as nitrate and sulfate.

Instream biogeochemical process measurements are often short-term and localized. Here we use in situ sensors to quantify the net effects of biogeochemical processes on seasonal patterns in baseflow nitrate retention at the river-reach scale. Dual-station high-frequency in situ nitrate measurements, were coupled with high-frequency measurements of stream metabolism and dissolved inorganic carbon, in a tributary of the Buffalo National River, Arkansas. Nitrate assimilation was calculated from net primary production, and combined with mass-balance measurements, to estimate net nitrification and denitrification. The combined net effects of these instream processes (assimilation, denitrification, and nitrification) removed >30–90% of the baseflow nitrate load along a 6.5 km reach. Assimilation of nitrate by photoautotrophs during spring and early summer was buffered by net nitrification. Net nitrification peaked during the spring. After midsummer, there was a pronounced switch from assimilatory nitrate uptake to denitrification. There was clear synchronicity between the switch from nitrate assimilation to denitrification, a reduction in river baseflows, and a shift in stream metabolism from autotrophy to heterotrophy. The results show how instream nitrate retention and downstream delivery is driven by seasonal shifts in metabolic pathways; and how continuous in situ stream sensor networks offer new opportunities for quantifying the role of stream biota in the dynamics, fate, and transport of nitrogen in fluvial systems.

In northwestern Arkansas, karst aquifers have experienced degraded water quality due to impacts of land-use changes, such as increased urbanization or intensification of agriculture. Water quality at a karst spring was characterized by quantifying the variability of fecal-indicator bacteria, specifically Escherichia coli (E. coli), throughout base-flow periods and storm events. Water-quality change over time was assessed using an intermittent record of nitrate and chloride data since the 1990s. A Geographic Information System was used to quantify changes in land use over time and identify housing subdivisions using on-site septic systems. E. coli was significantly greater during storm events (649 CFU/100 mL or MP/100 mL) than base-flow periods (41 CFU/100 mL or MPN/100 mL). The increase in E. coli at the spring following storm events was interpreted to result from flushing of bacteria from the land surface, through the epikarst and karst conduits, and into groundwater. Nitrate and chloride have increased significantly since the 1990s, following the general pattern of increased urbanization in the recharge area. Septic-tank effluent may be degrading the water quality of the karst spring based on the dominance of on-site septic tank usage in the recharge area, unsuitable topography and soil type for septic tank absorption fields, increased nitrate and chloride concentrations concomitant with increased urbanization, and increase of the fecal-indicator bacteria E. coli following storm events.

average values for d 26 Mg= � 0.832 � 0.068 and d 25 Mg= � 0.432 � 0.053 (n=40, 2SD) – identical to those of seawater from Hawaii (average d 26 Mg= � 0.829 � 0.037 and d 25 Mg= � 0.427 � 0.033) and to the average literature values of seawater worldwide (d 26 Mg= � 0.83 � 0.11 and d 25 Mg= � 0.43 � 0.06, n=49, 2SD). Collectively, global seawater has a homogeneous Mg isotopic composition with d 26 Mg= � 0.83 � 0.09 and d 25 Mg= � 0.43 � 0.06 (2SD, n=90). The magnesium isotopic composition of seawater is principally controlled by river water input, carbonate precipitation and oceanic hydrothermal interactions. The homogeneous Mg isotopic composition of seawater indicates a steady-state budget in terms of Mg isotopes in oceans, consistent with a long Mg residence time (~13 Ma). Considering that seawater is homogeneous, readily available in large amounts, can be easily accessed and processed for isotopic analysis, and has an isotopic composition near the middle of the natural range of variation, it is an excellent geostandard for accuracy assessment to rule out analytical artifacts during high-precision Mg isotopic analysis. Copyright © 2011 John Wiley & Sons, Ltd.

The potential health impact of As in drinking water supply systems in the Mississippi River Valley alluvial aquifer in the state of Arkansas, USA is significant. In this context it is important to understand the occurrence, distribution and mobilization of As in the Mississippi River Valley alluvial aquifer. Application of surface complexation models (SCMs) to predict the sorption behavior of As and hydrous Fe oxides (HFO) in the laboratory has increased in the last decade. However, the application of SCMs to predict the sorption of As in natural sediments has not often been reported, and such applications are greatly constrained by the lack of site-specific model parameters. Attempts have been made to use SCMs considering a component additivity (CA) approach which accounts for relative abundances of pure phases in natural sediments, followed by the addition of SCM parameters individually for each phase. Although few reliable and internally consistent sorption databases related to HFO exist, the use of SCMs using laboratory-derived sorption databases to predict the mobility of As in natural sediments has increased. This study is an attempt to evaluate the ability of the SCMs using the geochemical code PHREEQC to predict solid phase As in the sediments of the Mississippi River Valley alluvial aquifer in Arkansas. The SCM option of the double-layer model (DLM) was simulated using ferrihydrite and goethite as sorbents quantified from chemical extractions, calculated surface-site densities, published surface properties, and published laboratory-derived sorption constants for the sorbents. The model results are satisfactory for shallow wells (10.6 m below ground surface), where the redox condition is relatively oxic or mildly suboxic. However, for the deep alluvial aquifer (21–36.6 m below ground surface) where the redox condition is suboxic to anoxic, the model results are unsatisfactory.

Twenty one of 118 irrigation water wells in the shallow (25-30 m thick) Mississippi River Valley alluvial aquifer in the Bayou Bartholomew watershed, southeastern Arkansas had arsenic (As) concentrations (

Because meteoric water δ 18 O values decrease with decreasing ambient temperature and increasing latitude, δ 18 O values of meteoric calcite cement should exhibit a similar relation with paleolatitude and be an indicator of continental paleotemperatures. To test this, we compiled isotopic and paleolatitude data for 20 meteoric cements and nine speleothems ranging in age from Devonian to modern and in paleolatitude from 3.5° to 83°. Mean δ 18 O values for meteoric cements and speleothems both show the same negative correlation with paleolatitude. The δ 18 O vs. latitude trend for these carbonates is almost identical to that predicted for modern inland environments, but differs from the trend for coastal environments. This suggests that the ground water controlling the ultimate composition of meteoric cement is derived predominantly from inland recharge. If it is assumed that the modern meteoric water δ 18 O vs. temperature relation is valid for the past and that insignificant evaporation occurred prior to carbonate precipitation, then coastal and inland paleotemperatures can be calculated from the δ 18 O values of meteoric calcite (δ 18 O mcl ) and seawater (δ 18 O sw ) by using the equations T coastal =13.3 ±32.6[-0.231- 0.0613(δ 18 O mcl + δ 18 O sw ) ½ and T inland =17.8 ±16.2[-0.572 - 0.1233(δ 18 O mcl + δ 18 O sw ) ½ , where T is temperature in °C. Calcite precipitated from coastal meteoric water at temperatures between 0 and 25 °C will exhibit a narrow range in δ 18 O(-6‰ to -4‰, where δ 18 O sw = 0‰). The δ 18 O of calcite precipitated from inland meteoric water will be sensitive to paleotemperature, ranging from -14‰ to -5‰ (where δ 18 O sw = 0‰) for temperatures of 0 to 25 °C.