Noel C. Krothe

A four-component mixing model for water in a karst terrain in south-central Indiana, USA. Using solute concentration and stable isotopes as tracers

The study area lies in a highly karstified carbonate terrain in south central Indiana. Sinkholes, conduits, and caves form large secondary pathways for the subsurface flow. As a result, the discharge from a main emergence point for the subsurface flow system, the Orangeville Rise, quickly responds to the storm events and shows wide variations in flow rate, water chemistry, and stable isotopic compositions. These responses are attributed to the mixing of water in secondary pathways. In the study area, recharge occurs through the thick, mantled karst plain and the sinkhole plains, and the role of soil layer and epikarst in the recharge process is of great importance. Rain (DIC: 2 HCO3− mg/l, δ13CDIC: −7‰), soil water (DIC: 544 HCO3− mg/l, δ13CDIC: −14.7‰), epikarstic water (DIC: 224 HCO3− mg/l, δ13CDIC: −13.6‰), and phreatic diffuse flow water (DIC: 299 HCO3− mg/l, δ13CDIC: −11.8‰) generally showed unique and constant dissolved inorganic carbon (DIC) and δ13CDIC values over time. Using DIC and δ13CDIC as tracers, a four-component mixing model was established for the karstic flow system. By constructing the discharge hydrograph separation curves, the mixing ratio of each component, rain (10.6%), soil (3.1%), epikarstic (52.3%), and phreatic (34.0%) water, was determined for the Orangeville Rise discharge over the testing period of 104 h after the storm event of 10/4/90. Vadose water occupied 55.4% of spring discharge and this demonstrates the importance of the unsaturated zone, especially the epikarst, in the karstic flow systems.

Stable Isotopic Variation of Storm Discharge from a Perennial Karst Spring, Indiana

Oxygen and deuterium isotopes and major-ion chemistry of water from a large karst spring were used in an attempt to decipher water recharge, transmission, and storage characteristics of a karst aquifer system. Ionic concentrations and isotopic data indicated that the bulk of discharge during peak flow was derived from groundwater storage. Isotopic hydrograph separation of storm flow revealed that maximum rainwater contribution to discharge was 18 to 24 hours after peak flow and rainwater contributed 20 to 25% of spring discharge over the monitoring periods. Water released from phreatic and vadose conduit storage may have contributed to discharge with the onset of storm flow, while water from soil moisture and epikarst storage may have arrived during initial discharge recession.

Seasonal fluctuation in δ15N of groundwater nitrate in a mantled karst aquifer due to macropore transport of fertilizer-derived nitrate

Twenty wells were sampled and analyzed for nitrate content and nitrogen isotopes during late September, 1986. The groundwater nitrate δ15N values ranged from + 6.8 to + 17.6% with a mean of +10.3%. Eleven samples had δ15N values greater than +9.5% indicating an animal waste origin. Septic tanks are probably the predominant source of waste-derived nitrate. Macropores draining septic tank filter fields may provide a nearly continuous flow of effluent to the aquifer. The δ15N values of the other nine samples ranged from +6.8 to 8.4% suggesting a possible mixing of waste-derived nitrate and that derived from isotopically lighter cultivation sources, i.e. inorganic fertilizer and/or mineralized soil organic nitrogen. The twenty wells were resampled during late May, 1987 shortly after inorganic fertilizers had been applied to the corpland throughout the study area. The groundwater nitrate δ15N values ranged from +3.6 to + 14.7% with a mean of + 8.3%. Nineteen of the wells showed a shift towards the lighter δ15N values expected from the fertilizer-derived nitrate. The °15N decreases ranged from 0.6 to 4.8% with a mean of 2.2%. Despite the apparent influx of fertilizer-derived nitrate, eleven samples (with δ15N + 8.0 to + 14.7%) still were composed of predominantly waste-derived nitrate. Only nine samples (with °15N + 3.0 to + 5.8%) may have had fertilizer-derived component approaching in magnitude that of waste-derived nitrate. Regional groundwater nitrate concentration did not substantially increase. Macropore (cracks, root channels, etc.) flow appears to contribute significantly to the recharge of the aquifer. Rapid response of the groundwater nitrate δ15N values to fertilization demonstrates the ability of macropore flow to transport fertilizer-derived nitrate which has been flushed from the tilled soil layer. Other agriculture chemicals (pesticides and herbicides) may also be transported to the aquifer in this manner.

Infiltration mechanisms related to agricultural waste transport through the soil mantle to karst aquifers of southern Indiana, USA

Abstract A hydrogeological study was conducted, during the 1991–1992 water year, in the clay-soil mantled portion of a limestone terrain in southern Indiana. The purpose of the study was to investigate the modes of soil-water infiltration contributing to rapid transport of nitrate to the saturated zone. The 1-year-cycle profiles of nitrate concentration vs. time show a consistent increase of nitrate at various depths in the unsaturated zone during the period of investigation. The increase of nitrate in soil water is attributed to the rapid flushing of the inorganic fertilizers from the fields after the area received sufficient rainfall in late fall. The investigation also showed a major movement of nitrate in quick pulses through the unsaturated zone, rather than a slow uniform recharge, immediately after a major storm event. The asymmetric profiles of nitrate concentration vs. depth point to the existence of preferential flow through macropores in the clay-soil mantle above the bedrock. Soil-water transport between storm events is by matrix type flow. Nitrogen isotopes were analyzed for representative groundwater samples collected before and immediately after fertilization of fields in the summer, 1991. The 615N values of the samples did not show any major shift in nitrate sources between the sampling periods. The summer of 1991 was extremely dry prohibiting vertical transport of nitrate from the fields to the groundwater system. Any change in nitrate concentration in groundwater during this time is attributed to the mixing through lateral flow within the aquifer.

Sulfur isotopes and hydrochemical variations in spring waters of southern Indiana, U.S.A.

Abstract Water chemistry and δ 34 S(SO 4 ) studies suggest two flow systems in the karst terrane of southern Indiana: (1) a shallow flow system exists, which is dominated by surface water entering the ground through fractures; and (2) a deeper regional groundwater flow system recharged by diffuse flow. The chemistry of the water in the regional system may be dominated by the solution of gypsum in the Lower St. Louis Limestone. Sulfur-isotope studies suggest two flow systems and show that the high SO 2 − 4 concentrations in local waters result from solution of gypsum. Reported δ 34 S(SO 4 ) for Upper Mississippian evaporites and fresh water range from +14 to +19 to +8 to +12‰, respectively. Isotope analysis of gypsum cores from the Lower St. Louis Limestone evaporite unit shows δ 34 S-values between + 14.10 and + 15.13‰ in the study area. Groundwater chemistry studies show a direct linear relationship between SO 2 − 4 concentrations and δ 34 S-values. Groundwater varies in SO 2 − 4 concentrations between 20 and 1970 mg l −1 . The δ 34 S-values range from + 10.61‰ for a conduit spring to + 18.57‰ for a diffuse spring. The waters with high SO 2 − 4 concentrations have δ 34 S(SO 4 )-values higher than the local gypsum deposits analysed and contain H 2 S, suggesting a deeper flow system in which fractionation by bacterial reduction of SO 2 − 4 is occurring. Waters with low SO 2 − 4 concentrations have δ 34 S(SO 4 )-values in the range of local fresh water, indicating a shallow flow system.

Delineating the karstic flow system in the upper Lost River drainage basin, south central Indiana: using sulphate and δ34SSO4 as tracers

Abstract A karstic flow system in the upper Lost River drainage basin in south central Indiana, USA, was investigated using SO4 concentration and δ34SSO4 as tracers. The flow system was characterized as vadose flow and phreatic diffuse flow. Vadose-flow samples were collected from 7 epikarstic outlets after storm events. Phreatic diffuse flow samples were collected from the Orangeville Rise, the major emergence point for the drainage basin, during the base flow periods. Discharge from the Orangeville Rise was constant during the base flow periods but showed large variations in flow rate (0.3–11.7 m3/s), SO4 concentration (11–220 mg/l), and δ34SSO4 (+5.2 to +15.0‰) after storm events, due to the mixing of rain, vadose flow, and phreatic diffuse flow in the conduits that feed the Orangeville Rise. Sulphate concentrations and δ34SSO4 were unique in vadose flow (SSO4: 13–24 mg/l; δ34SSO4: +1.9 to +3.8‰) and phreatic diffuse flow (SO4: 220 mg/l; δ34SSO4: +15.0‰). Mean SO4 concentration of rainwater in the study area was measured as 1.8 mg/l. Using a 3-component mixing model for water in the karstic conduits, the mixing ratios of rain (16.5%), vadose flow (58.5%), and phreatic diffuse flow (25.0%) components were calculated in the Orangeville Rise discharge. These mixing ratios attained using SO4 concentration as a tracer indicated the important role of the vadose zone as a water storage area in karst aquifers.

δ15N of nitrate derived from explosive sources in a karst aquifer beneath the Ammunition Burning Ground, Crane Naval Surface Warfare Center, Indiana, USA

Abstract Military institutions involved in the production and demolition of explosives, propellants, and pyrotechnics have the potential to degrade groundwater aquifers through the addition of numerous contaminants including nitrate. A nitrate plume has been identified in a karst aquifer beneath the Ammunition Burning Ground (ABG) at the Crane Naval Surface Warfare Center, Indiana, USA. Wells located in the vicinity of surface impoundments and burn pans used for treatment of explosive materials show the highest concentrations of nitrate ranging from 11.2 to 19.6 mg 1−1 as NO3−. Little is known about the isotopic composition of nitrates originating from these processes. Eight wells within the ABG were sampled and analyzed for nitrogen isotopic composition of nitrate. An enrichment in the δ15N (δ15N = +8.9, +12.0, +13.1, and +13.5‰) occurred at four wells located near the primary areas of disposal activities within the ABG. Four wells located near the outer limits of the ABG had δ15N values significantly lower than those observed in the central area of the ABG (δ15N = +4.0, +4.1, +4.6, and +2.0‰). Soil samples and burn-pan ash samples were collected and analyzed for the nitrogen isotopic composition of nitrate. Three soil nitrate samples had low δ15N values of −1.7, −1.8, and +2.2‰. The burn-pan ash sample produced nitrate with a δ15N value of +2.9‰. The observed enrichment in δ15N from samples taken from wells located near the ABG has been postulated to be a result of photodegradation or biochemical modification of RDX and TNT contaminated sludges and volatilization of NH3 in storage lagoons within the ABG.