Acidic Groundwater Research Articles

Characterising the hydrothermal alteration of the Broadlands–Ohaaki geothermal system, New Zealand, using short-wave infrared spectroscopy

Hydrothermal clay minerals present in the Broadlands–Ohaaki geothermal field were characterised by field portable short-wave infrared spectroscopy. Three major alteration zones, an upper smectite, a middle illite and a lower illite–chlorite, are spectrally separable. The zoning pattern is generally consistent with the thermal structure of the geothermal field, although occasionally zone boundaries cut present-day isotherms. The data indicate that temperature is the major control on clay zoning and permeability plays a subordinate role. Both beidellite and montmorillonite are common in the upper, low-temperature smectite zone. Kaolinite, mainly of low crystallinity, marks the margin of the field where cool acidic ground waters inflow. In the middle alteration zone, illite, dominantly K-rich, shows a narrow compositional variability. Some highly permeable zones are characterised by illite with low octahedral Al contents. Ammonium-bearing illite and buddingtonite are present locally in permeable horizons within the illite zone, where temperatures are above 200°C. Chlorite is most abundant in the lower alteration zone (temperature >250°C), although it also occurs unevenly in the upper and middle alteration zones. Chlorite varies from Mg- to Fe-rich varieties (but mostly with Mg# values <0.5), but no compositional trends with respect to depth are spectrally detectable.

Stability constants for mono- and dioxalato-complexes of Y and the REE, potentially important species in groundwaters and surface freshwaters

We present the first measured set of stability constants for mono- and dioxalato-complexes of yttrium and all rare earths except Pm (Y+REE), Oxβ n = [MOx n 3−2n] [M 3+] −1 [Ox 2−] −n(where [ ] ≡ concentrations, M ≡ Y+REE, and Ox 2− ≡ C 2O 4 2−). Aqueous solutions of Y+REE were titrated with oxalic acid in the presence of a cation-exchange resin, and Y+REE concentrations in the solution phase were measured by ICP-MS. This method allows investigation of all Y+REE simultaneously under identical conditions and is thus very sensitive to subtle inter-element variations in log Oxβ n. Experiments were performed at a single ionic strength ( I = 0.05 M), but at two values of pH. Patterns of log Oxβ 1 and log Oxβ 2, determined from our experiments, are similar in shape and reminiscent of those for carbonato-complexes. The average ratio of stepwise stability constants K 2/K 1 = Oxβ 2/( Oxβ 1) 2 is 0.05 ± 0.02 for Y+REE excluding La and Ce. Literature values of Oxβ 1(Eu) for 0.03 mol/L ≤ I ≤ 1 mol/L were used to derive the relation log Oxβ 1(Eu) = log Oxβ 1 0(Eu) − 6.132√ I/(1 + 1.47√ I) + 0.902 I, where log Oxβ 1 0(Eu) is the stability constant at infinite dilution. Applying this relation to all Y+REE, the following values of log Oxβ 1 0 (at zero ionic strength) were found: 6.66 (Y), 5.87 (La), 5.97 (Ce), 6.25 (Pr), 6.31 (Nd), 6.43 (Sm), 6.52 (Eu), 6.53 (Gd), 6.63 (Tb), 6.74 (Dy), 6.77 (Ho), 6.83 (Er), 6.89 (Tm), 6.95 (Yb), 6.96 (Lu). These values, which are based on direct measurements for each individual Y+REE, agree quite well with published extrapolations that are mostly based on linear free-energy relationships. Total oxalate concentrations of 10 −5−10 −3 M have been reported for soil solutions. Free oxalate ions persist at much lower pH than free carbonate ions and a simple speciation model demonstrates that oxalato-complexes can dominate Y+REE speciation in mildly acidic groundwaters of low-to-moderate alkalinity.

Geochemical models of the impact of acidic groundwater and evaporative sulfate salts on Boulder Creek at Iron Mountain, California

During dry season baseflow conditions approximately 20% of the flow in Boulder Creek is comprised of acidic metals-bearing groundwater. Significant amounts of efflorescent salts accumulate around intermittent seeps and surface streams as a result of evaporation of acid rock drainage. Those salts include the Fe-sulfates — rhomboclase ((H3O)Fe3+(SO4)2·3H2O), ferricopiapite (Fe3+5(SO4)6O(OH)·20H2O), and bilinite (Fe2+Fe23+(SO4)4·22H2O); Al-sulfates — alunogen (Al2(SO4)3·17H2O) and kalinite (KAl(SO4)2·11H2O); and Ca- and Mg-sulfates — gypsum (CaSO4·2H2O), and hexahydrite (MgSO4·6H2O). The dissolution of evaporative sulfate salt accumulations during the first major storm of the wet season at Iron Mountain produces a characteristic hydrogeochemical response (so-called “rinse-out”) in surface waters that is subdued in later storms. Geochemical modeling shows that the solutes from relatively minor amounts of dissolved sulfate salts will maintain the pH of surface streams near 3.0 during a rainstorm. On a weight basis, Fe-sulfate salts are capable of producing more acidity than Al- or Mg-sulfate salts. The primary mechanism for the production of acidity from salts involves the hydrolysis of the dissolved dissolved metals, especially Fe3+. In addition to the lowering of pH values and providing dissolved Fe and Al to surface streams, the soluble salts appear to be a significant source of dissolved Cu, Zn, and other metals during the first significant storm of the season.

Effect of pH on Sorption and Transport of Fluorobenzoic Acid Ground Water Tracers

AbstractSorption and transport behavior of fluorobenzoic acid (FBA) tracers were evaluated as ground water tracers in a highly weathered and fractured clay‐rich saprolite under both acidic and neutral conditions in laboratory batch tests and column transport studies. The FBAs are a group of 14 organic acids with pKa from 2.71 to 3.83, and may thus be potentially retarded in weathered, acidic soils. The degree of sorption was directly related to the pKa of the isomer and the pH of the solution. A technique for determining the intrinsic partitioning coefficient (Kd) of an non‐ionized species was derived to predict the equilibrium sorption of a family of compounds over a range of pH and pKa. For the diflourobenzoic acids used in this study, Kd was 2.50 L/kg. The intact column transport experiments confirmed that at neutral pH, none of the FBA isomers were retarded relative to a nonreactive tracer (Br−). Under acidic conditions (pH = 5.2), significant retardation was observed for isomers with higher pKa. Because the use of FBA tracers could be limited by regulatory concerns about potential toxic effects in surface water receiving discharge from ground water containing FBA, aquatic toxicity was assessed using a Ceriodaphnia 96‐h acute toxicity test. The LD50 (the concentration resulting in mortality of ≥50% of the test organisms) for the four FBA isomers tested was above 100 mg/L. The results of these equilibrium isotherm and column transport experiments demonstrate that FBAs can be very useful as nonreactive ground water tracers in these materials as long as the pH is approximately 2 pH units above the pKa (i.e., &gt;99% ionized).

Sulfate Pools in the Weathered Substrata of a Forested Catchment

The mitigating effect of decreasing anthropogenic SO4 deposition on acidified soils and waters can be delayed by the release of previously stored soil SO4. We investigated SO4 pools and desorption in the weathered substrata (0.5–10 m depth) of a forested catchment on granite to quantify the importance of these layers to SO4 dynamics. Solid‐phase materials from 10 boreholes to a maximum depth of 10 m were analyzed for water‐ and phosphate‐extractable SO4, SO4 desorption, cation‐exchange capacity (CEC), pH, and dithionite‐ and oxalate‐extractable Fe (Fed and Feo) and Al (Ald and Alo). Seven of the investigated boreholes were used to monitor water table depth and to obtain samples for measurement of solution SO4 concentrations. The storage of phosphate‐extractable SO4 in the weathered substrata was estimated at 90 kmol ha−1, of which ≈50 kmol ha−1 were water soluble. Sulfate pools and their desorption behavior were highly variable, which could partly be explained by the variation of pH and extractable Fe and Al contents of the samples. Sulfate concentrations in groundwater were dependent on the depth of groundwater table and corresponded with the depth gradients of solid‐phase SO4. The SO4 pools of the substrata were apparently regulating solution concentrations. Thus, groundwater acidification in such aquifers will not be easily reversed by decreasing SO4 deposition because of the release of previously stored SO4.

Supergene Ferromanganese Wad Deposits Derived from Permian Karoo Strata along the Late Cretaceous-Mid-Tertiary African Land Surface, Ryedale, South Africa

A previously unknown type of ferromanganese wad deposit is described at the Ryedale mine situated 110 km west of Johannesburg in the northwestern province of South Africa. The wad was derived from supergene alteration of Glossopteris -bearing Permian strata of the Karoo Supergroup that fill shallow karstic depressions in Neoarchean Malmani dolomite of the Transvaal Supergroup. The depressions contain up to several million tons of friable and highly porous ferromanganese wad with an average Mn/Fe ratio of about 0.3 and high grades of between 77 and 91 wt percent Fe2O3 + MnO. A detailed mineralogical, petrographical, and geochemical study of the orebody suggests that the wad is a saprolitic residue of manganiferous blackband iron ores known to be associated with Permian coal measures of the Karoo Supergroup. The protore was deposited in shallow lakes in preexisting karstic depressions in the Malmani dolomite as finely laminated mud composed of biogenic detritus and Mn-Fe oxyhydroxide precipitates. Siliciclastic detritus is conspicuously absent, suggesting that the lakes were exclusively fed by reducing and acidic ground water that leached manganese and iron from the underlying Malmani dolomite. Anaerobic early diagenesis led to the transformation of oxide precipitates into Mn-Fe carbonates. Much later, during the African cycles of erosion and weathering, the blackband ores were exhumed, partly eroded, and altered by deep lateritic weathering to form the present ferromanganese wad.

Modeling Removal of Cd, Cu, Pb, and Zn in Acidic Groundwater during Neutralization by Ambient Surface Waters and Groundwaters

Removal of Pb, Cu, Zn, and Cd during neutralization of acid rock drainage is examined using model simulations of field conditions and laboratory experiments involving mixing of natural drainage and surface waters or groundwaters. The simulations consider sorption onto hydrous Fe and Al oxides and particulate organic carbon, mineral precipitation, and organic and inorganic solution complexation of metals for two physical systems where newly formed oxides and particulate organic matter are either transported or retained along the chemical pathway. The calculations indicate that metal removal is a strong function of the physical system. Relative to direct discharge of ARD into streams, lower metal removals are observed where ARD enters streamwaters during the latter stages of neutralization by ambient groundwater after most of the Fe has precipitated and been retained in the soils. The mixing experiments, which represent the field simulations, also demonstrated the importance of dissolved metal to particle Fe ratios in controlling dissolved metal removal along the chemical pathway. Finally, model calculations indicate that hydrous Fe oxides and particulate organic carbon are more important than hydrous Al oxides in removing metals and that both inorganic and organic complexation must be considered when modeling metal removal from aquatic systems that are impacted by sulfide oxidation.

Potential relative future effects of sulfur and nitrogen deposition on lake chemistry in the Adirondack Mountains, United States

Leaching of atmospherically deposited nitrogen from forested watersheds can acidify lakes and streams. Using a modified version of the Model of Acidification of Groundwater in Catchments, we made computer simulations of such effects for 36 lake catchments in the Adirondack Mountains of New York State. Our simulations bound the potential changes in regional‐scale surface water acidification that might occur there over the next 50 years across broad scenarios of both nitrogen and sulfur deposition as well as ranges of times to watershed nitrogen saturation. Model projections indicated that nitrogen deposition may play a more important role in future acidification than has been previously considered and may rival or exceed potential effects of sulfur deposition, depending on how rapidly watersheds might become saturated with atmospherically deposited nitrogen.

Relationship between landscape characteristics, history, and lakewater acidification in the Adirondack Mountains, New York

Interactions between acidic deposition and watershed characteristics were evaluated for a group of lakes in the Adirondack Mountains, New York. Landscape characteristics were compiled and examined relative to paleolimnological inferences of historical acidification. Results of estimates of acidification using the Model of Acidification of Groundwater in Catchments (MAGIC) and paleolimnological analysis were compared to physical, biological, and landscape change data, including such factors as watershed disturbance, logging, fire, and windthrow, to evaluate if inclusion of additional processes could improve model estimates. Results of bivariate and multivariate analysis confirmed that lakes that have experienced historical acidification tend to be those that receive relatively high amounts of precipitation and have short hydraulic residence times. These variables explained 58% of the diatom-inferred acidification. A combined model of long-term precipitation amount, hydraulic residence time, and recent blowdown accounted for 71% of the historic acidification in the Adirondacks. Lakes that have increased in pH since pre-industrial times tend to be those subject to substantial human disturbance and those that burned during major fires recorded after 1900. The magnitude of the discrepancy between MAGIC model and diatom-inferred hindcasts of acidification was not significantly correlated with any of the landscape change variables, suggesting that additional modifications to the MAGIC model to take into account landscape change are not likely to appreciably improve model performance.

A comparison of methods for estimating soil characteristics in regional acidification models; an application of the MAGIC model to Scotland

Abstract. The results from an application of MAGIC (Model of Acidification of Groundwater In Catchments) to 733 Scottish catchments are presented. The availability of representative, good quality soil data is frequently limiting factor for biogeochemical modelling, particularly those involving modelling at various spatial scales. This study tests the sensitivity of MAGIC to soil input data derived from two different methodologies; the "nearest neighbour method" considers the closest representative soil profile to a catchment, and the "spatial weighting method" of all soil types identified in a catchment, based on a soil physico-chemical classification of Scotland. Soil data (soil depth, density, cation exchange capacity and base saturation) calculated using the "nearest neighbour method" and the "spatial weighting method" were highly variable, although the range of upper and lower limits were greater for soil data produced using the nearest neighbour method. In contrast to the predominantly organic soil data calculated by the nearest neighbour method, the spatially weighted soil parameters included a greater proportion of mineral soils. With regard to simulated surface water Acid Neutralising Capacity (ANC) for 1851, 1997 and 2050, MAGIC predictions were similar irrespective of the methodology used to determine soil input parameters. However, soil input data derived from both methods resulted in variable base saturation predictions. It is concluded that the "nearest neighbour" methodology is most appropriate if the objective is to determine the predicted response of the most acid- sensitive sites within a region in line with the approach used in Critical Laod mapping. On the other hand, "spatial weighting" integrates catchment soils and represents a more robust methodology by which to determine changes in median soil and water response in a regional context. The anticipated reductions in S emissions associated with the Second S Protocol are predicted to have a marginal beneficial effect on the recovery of soils and surface waters of Scotland.

Regional modelling of acidification in Wales; calibration of a spatially distributed model incorporating land use change

Abstract. The Model of Acidification of Groundwater In Catchments (MAGIC) has been calibrated to 94 catchments in Wales, enabling the quantification of regional response to reduced emissions under the second sulphur protocol and incorporating the potential impact of future changes in afforestation. To calibrate the model the methodology utilises, for each site, the best available data derived from databases which are consistent across the whole region but which vary in spatial resolution. The calibrated model closely matches observed stream water chemistry across the region. The predicted future response is a reduction in acidity of surface waters in the region. The recovery is less marked at forested catchments, and some forested sites undergo a further decrease in acid neutralising capacity (ANC). The predictions from the multiple-site method are statistically consistent with those produced by an alternative regional modelling method which is based upon Monte-Carlo procedures. The multiple-site method, however, has the advantage of providing future predictions which are also spatially distributed across the region.

A regional application of the MAGIC model in Wales: calibration and assessment of future recovery using a Monte-Carlo approach

Abstract. A survey and resurvey of 77 headwater streams in Wales provides an opportunity for assessing changes in streamwater chemistry in the region. The Model of Acidification of Groundwater In Catchment (MAGIC) has been calibrated to the second of two surveys, taken in 1994-1995, using a Monte-Carlo methodology. The first survey, 1983-1984, provides a basis for model validation. The model simulates a significant decline of water quality across the region since industrialisation. Agreed reductions in sulphur (S) emissions in Europe in accordance with the Second S Protocol will result in a 49% reduction of S deposition across Wales from 1996 to 2010. In response to these reductions, the proportion of streams in the region with mean annual acid neutralising capacity (ANC) &gt; 0 is predicted to increase from 81% in 1995 to 90% by 2030. The greatest recovery between 1984 and 1995 and into the future is at those streams with low ANC. In order to ensure that streams in the most heavily acidified areas of Wales recover to ANC zero by 2030, a reduction of S deposition of 80-85% will be required.

Groundwater Exploitation and Development: Some Current Issues in Scotland

AbstractThe increasing cost of water in public supply is providing renewed interest in groundwater from industry and other intensive water‐users. At present, groundwater in Scotland enjoys considerable advantages: over surface water. However, there are also disadvantages: in some areas groundwater quality is poor due either to anthropogenic activity or the natural occurrence of acidic groundwaters. The future groundwater development and management policy for Scotland is being formed around the Water Resources Framework Directive and the Groundwater Action Programme. However, there are several policy constraints which urgently need to be addressed; these include (a) the role of the Water Order and (b) the need for a licensing system to control abstraction from selected aquifer units.

In-situ alteration of minerals by acidic ground water resulting from mining activities: preliminary evaluation of method

The chemical composition of the Cu-mining-related acidic ground water (pH ∼ 3.5 to near neutral) in Pinal Creek Basin, Arizona has been monitored since 1980. In-situ experiments are planned using alluvial sediments placed in the ground-water flow path to measure changes in mineral and chemical composition and changes in dissolution rates of subsurface alluvial sediments. The test results should help refine developed models of predicted chemical changes in ground-water composition and models of streamflow. For the preliminary test, sediment from the depth of the well screen of a newly drilled well was installed in three wells, the source well (pH 4.96) and two up-gradient wells (pHs 4.27 and 4.00). The sediment was placed in woven macrofilters, fastened in series to polyvinyl chloride (PVC) pipes, and hung at the screened level of each well. After interacting with the slowly moving ground water for 48 days, the test sediments were removed for analysis. There was no evidence that any of the materials used were biologically or chemically degraded or that the porosity of the filters was diminished by ferric hydroxide precipitation. These materials included 21-μm-pore (21PEMF) and 67-μm-pore polyester and the 174-μm-pore fluorocarbon Spectra/mesh macrofilters containing the in-situ sediment, the polypropylene (PP) macrofilter support structures, and the Nylon (NY) monofilament line used to attach the samples to the PVC pipe. Based on chemical and mineral composition and on particle-size distribution of the sediment before and after ground-water exposure, the 21PEMF macrofilter was chosen as the most suitable macrofilter for the long-term in-situ experiment. Tests also showed that the PP support structures and the NY monofilament line were sufficiently durable for this experiment.

Application of geologic map information to water quality issues in the southern part of the Chesapeake Bay watershed, Maryland and Virginia, eastern United States

Geologic map units contain much information about the mineralogy, chemistry, and physical attributes of the rocks mapped. This paper presents information from regional-scale geologic maps in Maryland and Virginia, which are in the southern part of the Chesapeake Bay watershed in the eastern United States. The geologic map information is discussed and analyzed in relation to water chemistry data from shallow wells and stream reaches in the area. Two environmental problems in the Chesapeake Bay watershed are used as test examples. The problems, high acidity and high nitrate concentrations in streams and rivers, tend to be mitigated by some rock and sediment types and not by others. Carbonate rocks (limestone, dolomite, and carbonate-cemented rocks) have the greatest capacity to neutralize acidic ground water and surface water in contact with them. Rocks and sediments having high carbon or sulfur contents (such as peat and black shale) potentially contribute the most toward denitrification of ground water and surface water in contact with them. Rocks and sediments that are composed mostly of quartz, feldspar, and light-colored clay (rocks such as granite and sandstone, sediments such as sand and gravel) tend not to alter the chemistry of waters that are in contact with them. The testing of relationships between regionally mapped geologic units and water chemistry is in a preliminary stage, and initial results are encouraging.

Analysis and simulation of reactive transport of metal contaminants in ground water in Pinal Creek Basin, Arizona

Large-scale mining activities have generated a plume of acidic ground water more than 15 km long in the regional aquifer of the Pinal Creek Basin. A one-dimensional reactive-transport model was developed using PHREEQC to aid in the analysis of transport and chemical processes in the plume and to determine the uses and limitations of this type of modeling approach. In 1984, the acidic part of the plume had a pH as low as 3.4 and contained milligram-per-liter concentrations of iron, copper, aluminum and other metals. From 1984 to 1994, concentrations of contaminants in the alluvial aquifer in Pinal Creek Basin, Arizona, decreased as a result of mixing, recharge, remedial pumping and chemical reactions. For reactions involving gypsum and rhodochrosite, the equilibrium modeling assumption of a local geochemical equilibrium was generally valid. From 1984 to 1990, water along the simulated flow path was at equilibrium or slightly supersaturated with gypsum, and gypsum equilibria controlled dissolved concentrations of calcium and sulfate. Beginning in 1991, water in the acidic part of the plume became increasingly undersaturated with respect to gypsum, indicating that the gypsum available for dissolution in the aquifer may have been completely consumed by about 1991. Rhodochrosite precipitation was thought responsible for the measured attenuation in dissolved manganese in the neutralized zone. For reactions involving calcite, the assumption of a local geochemical equilibrium was generally not valid. Dissolution of calcite in the transition zone was not sufficient to establish equilibrium although, following neutralization, the calcite saturation index decreased to −1.2 in 1986. Calcite undersaturation decreased along the flow path in the neutralized zone, and equilibrium was attained about 7 km downgradient of the transition zone. The assumption of a local geochemical equilibrium was not valid for oxidation–reduction reactions that involved iron oxides and manganese oxides. Kinetically controlled oxidation–reduction reactions continued in the acidic part of the flow path for years following the passage of the transition zone. Although the equilibrium approach helped to provide an increased understanding of contaminant transport at Pinal Creek, future work will require a kinetic modeling approach to more accurately simulate selected reactions between the plume and aquifer materials.

Modelling the effect of chemical heterogeneity on acidification and solute leaching in overburden mine spoils

The generation of acid mine drainage from overburden spoil piles at open-pit lignite mines is impacting the quality of groundwater and surface water bodies in large parts of the Lusatian mining area in Germany. Values of pH as low as 1 have been observed in the groundwater. After decommissioning, mine pits are generally converted to lakes which may also be acidic owing to the acidic groundwater discharge. The acidic effluent is generated by sulphide oxidation in the unsaturated zone of the spoil pile which generally extends to large depths as a result of dewatering. The long-term evolution of the acidification is still largely unknown. Our research focuses on the effects of physical and chemical heterogeneity caused by mixing of soil materials that may have already been oxidized to different degrees during the deposition of the spoil pile. Processes considered include variably saturated groundwater flow, oxygen diffusion in the soil gas, kinetic pyrite oxidation and acidic effluent generation, advective–dispersive transport of the aqueous components, equilibrium geochemical reactions between the chemical components and the soil minerals, and possible buffering and acid neutralization. Several existing numerical codes were coupled to represent the complete set of processes. Simulations were carried out in one- and two dimensions using representative characteristics of mine spoil piles, with the two-dimensional representation being based on spatially heterogeneous random fields of hydraulic conductivity and sulphide mineral fractions. Results show the long-term evolution of the oxidation front, the mass flux of oxidation products and the effects of system heterogeneity. Under conditions of constant flow, the system is found to return to neutral conditions over a time period on the order of several decades. Further work, including sensitivity analyses with respect to the controlling parameters and model calibration using site-specific field data, will be necessary to obtain a full understanding of the complex interacting mechanisms. The simulation approach shows promise as a potentially useful tool for predicting the effects of sulphide oxidation on the water quality of pit lakes.

Environmental impact of uncontrolled waste disposal in mining and industrial areas in Central Germany

The present-day landscape in Central Germany, in particular the region of Leipzig, Halle and Bitterfeld, is characterized by the scars of former industrial activities. Vast districts have been devastated by lignite strip mining. Industrial and domestic waste, residues from ore smelting, and highly toxic waste products from petrochemical plants and pesticide production were deposited in abandoned pits near population centers. The chief effects of waste on the environment are the contamination of groundwater by dissolved pollutants, the acidification of soil and water by the oxidation of pyrite-containing mining waste, and the salinization of shallow aquifers by rising brines from adjacent confined groundwater affected by mining. The consequences for the region are serious: mining lakes used for recreational purposes are contaminated by leachates from adjacent waste dumps. Pyrite-containing refuse from lignite mining under oxidation gives rise to the acidification of surface and groundwater, a basic condition for pollutant mobilization. In former metal mining districts, metalliferous and radioactive residues from smelting jeopardize public health. These effects are described in detail using three case studies.

Determining the Age of What Is Not There

How old is a cave? This is not an easy question. A cave is not an actual object but is rather the absence of material, and ascertaining the age of what is not there is difficult. In his research commentary, Sasowsky discusses results presented in the same issue by [ Polyak et al ][1]. in which a new strategy has been developed for dating caves created by the dissolving action of acidic ground water in limestone. In particular, the method has been applied to two of the most famous caves in the world: Carlsbad and Lechuguilla caves in New Mexico, USA. [1]: http://www.sciencemag.org/cgi/content/short/279/5358/1919

Reduced-Form Modelling of Surface Water and Soil Chemistry for the Tracking and Analysis Framework

A reduced-from modelling approach is used to predict soil and lake acidification as part of an integrated assessment of acid deposition effects and control strategies. The reduced-form model is based upon the mechanistic, lumped parameter watershed chemistry model, MAGIC (Model of Acidification of Groundwater in Catchments). Recent improvements to MAGIC are described, and its reduced-form representation in the Tracking and Analysis Framework (TAF), an integrated assessment model for decision and uncertainty analysis, is presented. Reduced-form models are developed for lake acid neutralising capacity (ANC), calcium, pH, and aluminium; and for soil base saturation. The model for lake ANC incorporates long-term depletion of the watershed acid neutralisation fraction, as is predicted to occur with MAGIC. In test calibrations for 33 representative watersheds in the Adirondacks, the resulting reduced-form model provides a close approximation to MAGIC, with average root mean square errors of 0.79 μeq l-1 for ANC, 1.09 μeq l-1 for calcium, 0.16 for pH, 2.52 μeq l-1 for aluminium, and 0.09% for soil base saturation. In addition, improved fish viability models are incorporated in the integrated assessment model, and predictions are demonstrated for a future deposition reduction scenario for the Adirondack region.