Mineral Controls in Colloid-Mediated Transport of Contaminants in Soil Environments
Heavy Metal Transport Mediated by Biosolid Colloids and their Mixtures with Soil Colloids
Pedogenic Processes in Karst Basins of Kentucky
Concentration and Distribution of Trace Metals in Northern Kentucky Soils
Metal Attenuation Processes in a Renovated Constructed Wetland Treating AMD
Geochemistry Gradients in a Coal-Mine Drainage Wetland Substrate
Biogeochemical Processes to Assess the Water Quality Improvement Function of Mitigated Wetlands
Mineral Controls in Colloid-Mediated Transport of Contaminants in Soil Environments
A. D. Karathanasis
Traditionally, hydrophobic environmental contaminants, such as heavy metals and herbicides, were assumed to be relatively immobile in subsurface soil environments because they are strongly sorbed by the soil matrix. However, under certain conditions their mobility may exceed ordinary rates and pose a significant threat to surface- and groundwater quality. Contamination risks may be accentuated by increasing anthropogenic inputs to terrestrial environments and potential contributions by mobile colloidal particles acting as carriers or facilitators of contaminant transport to groundwater (Grolimund et al., 1996; Kaplan et al., 1997). This threat has been substantiated by recent research evidence showing that water-dispersed colloidal particles migrating through soil macropores and fractures can significantly enhance contaminant mobility, causing dramatic increases in transported metal load and migration distances (Liang and McCarthy, 1995). Contrary to initial findings emphasizing organic colloids as the principal contaminant carriers (Kretzschmar and Sticher, 1997), more recent research has documented similar or greater potential for contaminant-facilitated transport by mineral colloid particles (Seta and Karathanasis, 1997; Karathanasis 1999, 2000; Karathanasis and Ming, 2002). The generally higher binding energies of hydrophobic contaminants for mineral- rather than organic- colloid surfaces make high surface charge mineral colloids more potent carriers of such contaminants (Bertsch and Seaman, 1999).
Materials and Methods
This study investigated the effect of colloid mineralogical composition on their capacity to mediate the transport of heavy metals and herbicides through undisturbed soil monoliths and field lysimeters, representing a Typic Paleudalf and a Typic Argiudoll with considerable macroporosity. In one of the laboratory experiments uncontaminated soil monoliths of 15-cm diameter and 20-cm length were leached with metal- (Cu, Zn, or Pb) or herbicide- (atrazine, metolachlor) contaminated suspensions of ex-situ soil colloids or biosolid colloids with diverse mineralogical composition at a concentration range of 100-300 mg/L. The soil colloids were separated from Bt soil horizons with montmorillonitic, illitic, mixed, or kaolinitic mineralogy, while the biosolid colloids were fractionated from a lime-stabilized municipal waste. In a second laboratory experiment the soil monoliths were contaminated with Pb (up to 60% of their sorption capacity) before they were flushed with uncontaminated soil colloids with montmorillonitic, illitic, and mixed mineralogy to asses their potential to desorb Pb previously retained by the soil matrix. All lab experiments involved continuous leaching cycles at a rate of 2.0 cm /hr, controlled by a peristaltic pump over 5-30 pore volumes. A third experiment involved leaching of field lysimeters, 50-cm in diameter and 100-cm in length, with indigenous soil colloid suspensions of 300 mg/L, contaminated with Zn and atrazine. Finally, in a fourth experiment a Zn-contaminated field lysimeter (up to 50% of sorption capacity) was flushed with uncontaminated soil colloids separated from Bt soil horizons with montmorillonitic mineralogy or O horizons with high organic carbon content to assess their potential to desorb Zn previously retained by the soil matrix. The field lysimeter experiments involved intermittent leaching cycles of 500 ml pulses at 8 hr intervals over 1.6-2.0 pore volumes. Metal or herbicide solutions containing soluble contaminant concentrations equivalent to the colloid suspensions were used as controls. Eluents were monitored for colloid, metal, and herbicide concentrations in the soluble and sorbed phase. Colloid-mediated transport interpretations were based on breakthrough curves of reduced colloid, metal, and herbicide concentrations (ratio of effluent to influent ratio) as a function of eluted pore volume. Experimental results were replicated within ±15% of the reported mean values.
Results and Discussion
Conclusions
The findings of this study document the important role of mineral colloid particles as contaminant carriers and facilitators in subsurface soil environments, even when the influence of organic constituents appears to be quite dominant. Depending on colloid mineralogical composition and charge properties, metal and herbicide loads transported in the presence of colloids may be several orders of magnitude greater than those transported by the soluble phase alone. Even though the stronger sorption affinity or binding energy of the contaminants for the mineral colloid surface appears to be the main facilitating transport mechanism, contributions by physical exclusion, competitive sorption, co-precipitation, and organic complexation processes may also increase contaminant transfer in the presence of colloids. In contaminated sites high surface-charge colloids may even strip contaminants from the soil matrix and remobilize them into subsurface environments. These results have important ramifications on modeling and prediction of contaminant transport and the application of suitable remediation technologies.
Heavy Metal Transport Mediated by Biosolid Colloids and their Mixtures with Soil Colloids.
C.M. Johnson, and A.D. Karathanasis.
This study investigated the role of colloid particles dispersed from various biosolid amendments to mediate the transport of associated metals through intact soil monoliths in laboratory leaching experiments. Water-dispersible colloids of <1 µm in size and concentrations of 100 mg/L were fractionated from one agricultural (poultry manure) and two municipal (lime-stabilized and aerobically digested biosolids) materials by centrifugation. Mixtures (50/50) of these biosolid colloids with soil colloids fractionated in the same manner from three soils with diverse physicochemical and mineralogical properties were also prepared to assess their role in mediating metal transport. Colloid composition ranged from 0-50% in carbonates, 5-15% in aluminosilicate minerals, 20-60% in organic carbon, and 2-10 mg g-1 in trace metals. Elution pH and EC conditions ranged from 4.0-8.0, and 6.0-70.0 uS/cm. The biosolid colloids and their soil colloid mixtures were applied to undisturbed soil monoliths of a Maury (Paleudalf), Woolper (Argiudoll), and a Bruno (Udifluvent) under steady rate (0.7 cm/hr) gravity flow conditions. Deionized water spiked with metals at levels similar to the total load (soluble plus sorbed) carried by the colloids was used as a control leaching treatment. The eluents were monitored for colloid, Cu, Zn, Pb, Cd, Cr, and Mo breakthrough concentrations. Biosolid colloid elution was mostly irregular, ranging from 0.0 to > 1.0 C/Co even for the same colloid within the same leaching cycle suggesting considerable physical and chemical interaction with the soil matrix during the leaching process. Increased levels of organic matter and surface area generally enhanced colloid elution, while elevated pH, carbonate content and increased colloid size inhibited colloid breakthrough. Mixing biosolid colloids with soil colloids also reduced overall colloid stability and elution. Total metal elution in the presence of colloids was several orders of magnitude greater than the control (soluble metal phase). In all cases, eluted total metal levels increased with colloid elution, suggesting a strong colloid-metal association, but the magnitude of metal elution was colloid- and soil specific. Under the most favorable conditions, average total metal elution in association with biosolid colloids ranged from 65 to 80% of the input metal concentration. Between 20 and 90% of the eluted metal load was colloid-bound, while the rest was soluble. The aerobically digested biosolid waste and the poultry manure were the most efficient metal carriers, thus posing the greatest risk for groundwater contamination. The strongest metal-colloid association was observed with Pb and the weakest with Mo, suggesting chemisorption to biosolid colloid surfaces as the main mechanism of mobilization for most metals and organic complexation as the main transport mechanism for Mo and a considerable soluble fraction of Cu and Zn. The addition of soil colloids to biosolid colloid suspensions moderated metal breakthrough patterns but maintained considerably higher metal elution levels than respective control treatments. These trends are consistent with stronger overall metal binding affinities exhibited by the biosolid colloids and their soil colloid mixtures over those of the soil matrices. The findings of this study suggest a greater potential mobility and contamination risk than originally anticipated from metals associated with the land application of biosolid wastes through colloid mediation and co-transport processes.
Pedogenic Processes in Karst Basins of Kentucky
Priyono Prawito and A. D. Karathanasis
The western Pennyroyal region of Kentucky is underlain by high purity Mississippian limestones with highly expressed karst topography. Studies are being conducted in typical karst basins to assess soil hydrology, deposition patterns, and karst basin stability. Major depositional events in two basins were evaluated from macro- and micromorphological observations, physicochemical properties and 14C dating. Sedimentation rates and karst basin stability interpretations were based on 14C dating and activities of 137Cs from nuclear fallout.
Morphological observations suggested the presence of buried soils. Thin section micromorphology evaluations confirmed the presence of buried soils (better pedality expression, brighter birefringence) in spite of the uniform silt loam texture throughout the excavated profile depths. Organic carbon content fluctuated irregularly with depth reflecting the age differences as well as the different land use management between time spans. Age discontinuities were evidenced by sharp decreases in the silt/medium sand (Si/MS), silt/fine sand (Si/FS), and CEC/Mg ratios in the Ab horizons. The vermiculite/mica (V/M) ratio was also a useful discontinuity indicator, but quartz content, quartz/feldspar (Q/F) and Ti/Zr ratios were not as consistent.
Soil development and thickness of the buried soils combined with 14C data suggested dramatic changes in the filling rate of the basins during modern time. Calculated sedimentation rates from 14C dating suggested a dramatic increase (20-50 fold)
14C-dated soil horizons (MRT) and estimated over the last 300 years. Most recent (last 50 filling rates (FR) of karst basins years) sedimentation rates based on 137Cs fallout activity appeared more moderate, but still 2-6 times higher than ancient timespans.
Characterization and Evaluation of High Elevation Wetlands in the Southern Appalachian Mountain Region of Kentucky
Brian Sandefur and A. D. Karathanasis
Wetlands located in the mountainous region of southeastern Kentucky represent unique ecosystems in the region. However, scientific study of these rare ecosystems has been limited due to their typical locations in rugged and inaccessible terrain. In this study, two wetlands located on Pine Mountain (Kentenia and Four Level) and a third site near Cumberland Mountain (Martins Fork) were investigated to observe geomorphology, hydrology, nutrient status, pore water geochemistry and plant community composition.
The wetland soils differed from site to site, but all were classified in the Aquept suborder. Both soils at Pine Mountain were classified as Endoaquepts, while the Martins Fork soil was found to be a Fluvaquentic Humaquept. Morphological descriptions of these soils revealed low chroma matrices and the presence of redox depletions or concentrations. Sandy loam textures dominated the profile at each site. Base saturation was low and ranged from 10% at Kentenia to 42% at Four Level.
Hydrology within the wetlands depended largely upon precipitation, seepage and runoff. Groundwater flow into the wetlands maintained saturated conditions throughout the year. However, precipitation events directly impacted hydrology by increasing water levels or temporarily inundating the wetlands during the wettest seasons. Reducing conditions were maintained throughout the year. Analysis of pore water chemistry indicated weakly minerotrophic conditions within each of the sites. Iron concentrations increased with depth, reflecting the more pronounced redox conditions with soil depth. Other ions, Ca, Mg, Mn, K, PO4-P and SO4-S, followed this trend, increasing in concentration with depth as these elements were released into solution following the reduction of iron minerals.
The plant communities within the wetlands were herbaceous with few scattered shrubs and small trees. Species diversity within the study sites did not vary significantly between the sites. The dominant plant species included Osmunda cinnamomea, Carex spp., O.regalis, Sphagnum and Trauteveria caroliniensis at Martins Fork; Sphagnum spp, Solidago uliginosa, Viola cucullata, O. cinnamomea, and Carex spp. at Kentenia; and Carex spp., Solidago ulginosa, Poa spp., Ipatiens capensis, and Polygonum sagtittatum at Four Level. Soil nutrients and wetland hydrology appeared to influence the prevalence of individual species.
Plant community at Martins Fork study site.
Automated well and weather station at the Kentenia site.
Concentration and Distribution of Trace Metals in Northern Kentucky Soils
Jutta Pils and A, D. Karathanasis
Elevated concentrations of trace metals are known to endanger ecosystems and jeopardize human health. Investigation of anthropogenic inputs of trace metals to soils in rapidly developing urban and suburban areas provides an opportunity to monitor environmental changes and degradation in response to human activities. Northern Kentucky, part of the Cincinnati, Ohio metro area, has developed rapidly in recent decades. Coal burning electric power plants, highway corridors, the Greater Cincinnati Airport, industrial plants and residential development are potential contributors to metal contamination of soils of the area. Twenty-six relatively undisturbed sites with soils of glacial, alluvial, or residual origin were selected for determination of background concentrations of Cd, Cr, Cu, Ni, Pb, and Zn. Soil samples from three depths; surface, middle B horizon, and C, were analyzed to determine concentrations of the metals.
Total metal analysis in three soil depths showed concentrations in the low to middle range of baseline concentrations reported for U.S. and world soils, suggesting a lack of significant anthropogenic inputs in the studied area. Zinc and Ni had the highest concentrations throughout the profile. Metal distribution in the soil profiles indicated Cr, Cu, Ni, and Zn increase with depth, but Cd and Pb levels remain stable throughout the profile. Organic matter, pH, and CEC or clay content were strongly associated with metal retention, with OM and pH being the dominant factors in the surface and middle soil depths. Correlation coefficients for all soil samples combined between metal content and soil parameters showed a strong pH effect for Cr, Pb, and Zn, and a prominent association of Pb and Zn with OM content. The amount of clay positively effected Cr and Ni retention, whereas silt showed a negative effect on Cr, Cu and Ni concentrations. Multiple regression analysis suggested OM as the most important soil variable influencing metal retention throughout the soil profile. Cation exchange capacity was the second most influential parameter, followed by clay content, and CEC/clay.
Alluvial soils contained significantly higher metal concentrations, especially Zn, compared to glacial till and residual soils, and higher Cr and Pb compared to glacial till soils. Elevated concentrations of Cr, Cu, Ni and Zn in surface horizons of alluvial soils may indicate metal enrichment through depositional processes, while metal increases with soil depth suggest parent material and pedogenic influences. No significant effects of fragipan horizon presence or soil-depth to bedrock on metal distribution were evident. Single correlations among metals showed close associations of Zn with Cd, Cr, Cu and Pb in all soil depths sampled.
Metal Attenuation Processes in a Renovated Constructed Wetland Treating AMD
Christopher Barton and A. D. Karathanasis
Acid mine drainage (AMD) from abandoned mines has significantly impaired water quality in eastern Kentucky. A small surface flow wetland constructed in 1989 to reduce AMD effects and subsequently failed after six months of operation was renovated by incorporating anoxic limestone drains (ALDs) and anaerobic subsurface drains promoting vertical flow through successive alkalinity producing limestone/organic compost beds. Two years of post renovation monitoring indicate that mean iron concentrations have decreased from 787 to 39 mg L-1, pH increased from 3.38 to 6.46and acidity has been reduced from 2244 to 199 mg L-1, (CaCO3 equivalent). Mass removal rates averaged 98% for Al, 95% for Fe, 94% for acidity, 55% for sulfate and 49% for Mn during the study period. The combination of ALDs and SAPS technologies used in the renovation and the sequence in which they were implemented within the wetland system appeared to provide sufficient buffering and longer residence time rendering a promising design for treatment of this and other source of high metal load AMD.
Characterization of sediments from
abiotic/aerobic zones within the treatment system showed low SO4/Fe ratios in initial abiotic treatment basins, supporting the formation of jarosite and goethite. As the ratios increased due to treatment and subsequent reductions in iron concentration, jarosite was transformed to other Fe-oxyhydroxysulfates and goethite formation was inhibited. Amorphous iron minerals such as ferrihydrite and Fe(OH)3 were dominant in biotic wetland cell substrates. However, low Fe3+ activity, redox potential, and oxygen diffusion rates in the anaerobic subsurface environment inhibited crystalline iron precipitation. The formation of gypsum, rhodochrosite, and siderite as byproducts of alkalinity-generating reactions in this system also appeared to have an impact on S, Mn, and Fe solubility controls. Sustaining alkaline conditions within the wetland was necessary for maintaining metal retention consistency and long-term treatment efficiency.
Fe-precipitates in AMD wetland flume
Vegetation Effects on Fecal Bacteria, BOD, and TSS Removal in Constructed Wetlands Treating Domestic Wastewater
Cora Potter and A. D. Karathanasis
Constructed wetlands have emerged as a viable alternative for secondary treatment of domestic wastewater in areas with landscape limitations, poor soil conditions, and high water tables, which limit installation of full-scale adsorption fields. Existing information on the effects of macrophytes on treatment performance is contradictory and mostly derived from greenhouse mesocosm experiments. This study investigated the removal efficiency of fecal bacteria, biological oxygen demand (BOD), and total suspended solids (TSS) in twelve constructed wetlands treating secondary effluent from single household domestic wastewater in Kentucky. The wetlands were monoculture systems planted to cattails (Typha latifolia) or fescue (Festuca arundinacea), polyculture systems planted with a variety of flowering plants, and unplanted systems. Influent and effluent samples were taken on a monthly basis over a period of one year and analyzed for fecal coliforms, fecal streptococci, BOD and TSS. The findings suggested no significant differences (P<0.05) in the average yearly removal of fecal bacteria (>93%) between systems, with the vegetated systems performing best during warmer months and the unplanted systems performing best during the winter. The vegetated systems showed significantly greater (P<0.05) removal efficiencies for BOD (>75%) and TSS (>88%) than the unplanted systems (63% and 46%, respectively) throughout the year. Overall, the polyculture systems seemed to provide the best and most consistent treatment for all wastewater parameters, while being least susceptible to seasonal variations. The performance of the cattail systems may improve by harvesting the plants at the end of the growing season, thus reducing additional BOD and TSS inputs from decaying biomass litter. The fescue systems were generally inferior to the polyculture and cattail systems because of their shallow rooting zone and limited biofilm surface area, while the unplanted systems were completely inefficient for BOD and TSS removal and should not be recommended.
A. D. Karathanasis and Brian Boone
In Kentucky contamination of surface and ground waters by fecal bacteria and nutrients originating from failing or inadequately designed septic systems has been a major non-point source pollution problem. As residential development encroaches upon rural areas and increasing population places additional demands upon the Commonwealth’s water resources the need for adequately functioning septic systems in the state will become more critical. Current guidelines assume that a minimum of 12” of suitable soil material between the bottom of the drain field and limiting soil features, such as bedrock, fragipan, claypan, unsuitable structure, or water table will provide adequate treatment to infiltrating sewage effluents. This vertical distance separation standard has never been experimentally tested under Kentucky soil conditions in spite of the fact that more than 60% of the soils in the state have at least one of these limitations. The objectives of this study were to evaluate the effectiveness of 12”, 18”, and 24” depth in representative Kentucky soils to treat house-hold septic effluent. The assessments were accomplished by leaching undisturbed soil monoliths (10” in diameter) representing the four different textural groups and hydraulic loadings recommended by the Kentucky Department for Public Health, with domestic wastewater effluent collected regularly from a house-hold septic system in Lexington. Eluent concentrations were monitored daily over a 15 day period for fecal bacteria, biochemical oxygen demand (BOD), total suspended solids (TSS), total-N, NH4-N, NO3-N, NO2-N, and total-P.
The results of the study indicate an alarming frequency of failure to comply with EPA criteria for most soils and wastewater quality parameters, especially for fecal bacteria, when using a 12” vertical separation distance between the bottom of the drain-field and a limiting soil interface. The treatment performance was especially poor, in coarse-textured soils with regard to fecal bacteria, BOD, total-N and NH4-N removal. Although biomat development over time is expected to improve treatment for some of these parameters, the high influent levels of fecal bacteria and BOD, pose great concerns for surface and groundwater contamination. Fine-textured soils generally provided better treatment efficiency, more consistent compliance for BOD, total-N, NH4-N, and total-P, and greater nitrification/denitrification potential. Treatment efficiency and compliance for most water quality parameters usually improved with increasing soil depth, with the 24” thickness providing the most consistent performance and compliance with MDL requirements. The small number of sites investigated hampered somewhat the statistical strength of some relationships, but the results suggested a controlling influence of soil type and textural group on treatment performance, and a general inadequacy of the 12” vertical separation distance to provide effective treatment of septic effluents, for most of the wastewater parameters and soils studied. In spite of considerable variability between and within soil groups, increasing the soil depth to 18” or 24” provided a better overall treatment for most wastewater parameters, particularly in coarse-textured soils. Considering that an increase in the current vertical separation distance standards will reduce even further the number of suitable soil sites in Kentucky, complementary or alternative treatment technologies should be adopted in marginal soils to improve treatment efficiency and prevent further deterioration of the quality of water resources in the state.
Geochemistry Gradients in a Coal-Mine Drainage Wetland Substrate
A. D. Karathanasis and J. E. Robinson
Coal-mine drainage has severely impacted water quality and imposed devastating effects on sensitive aquatic ecosystems world-wide. Constructed wetlands have been promoted as low maintenance cost-effective alternatives to conventional chemical treatments to achieve long-term treatment efficiency. Optimizing the efficiency of these systems requires successive aerobic and anaerobic treatment stages during which metal precipitation and acidity buffering can occur (Barton and Karathanasis, 1998). Although treatment efficiency assessments for the majority of cases has been based on short-term influent and effluent monitoring (Wieder, 1993), porewater geochemistry and sediment characterizations are important in order to better understand and control transformation processes and mechanisms dictating long-term treatment efficiency (Karathanasis and Thompson, 1995; Barton and Karathanasis, 1998; Bigham and Nordstrom, 2000; Gagliano et al., 2003). The objectives of this study were to monitor the surface and porewater geochemistry of a coal-mine wetland substrate and determine aerobic-anaerobic interfaces controlling solution composition and metal precipitation gradients.
Materials and Methods
This study was conducted in a constructed wetland treating a high metal and acidity load coal-mine drainage emanating from an underground mine in the Rock Creek watershed in southeastern Kentucky, USA. The system involves a set of five aerobic basin-vegetated SAPS treatment sequences, providing a treatment surface area of ~ 1022 m2 for a coal-mine drainage influent averaging hydraulic loads of 150 L/min, and metal/acidity loads of 800/2200 mg/L, respectively. Coal-mine drainage leaving the aerobic basins enters the cattail-planted SAPS cells via subsurface upward flow through a limestone gravel-compost substrate 45 cm deep. Substrate sediment core samples were taken with a bulk density sampler bearing 5 cm diameter plastic liners to a depth of 30 cm. Water samples from the surface water column and the substrate were collected with 40-cm long plexiglass porewater equilibrators, consisting of thirty-five 8 ml cells. Porewater samples were partitioned in three subsamples, one of which was used for pH, EC, acidity, alkalinity and sulfide determinations after purging with N2 gas, one was left as sampled for gas composition analysis, and the third was acidified with HCl for elemental analysis. Acidified samples were analyzed for total Fe, Al, Mn, and Ca by atomic absorption spectroscopy; and for Fe 2+, Fe3+, and SO42- colorimetrically. Sulfide concentrations were measured with a silver/sulfide ion selective electrode. Organic carbon, CO2, CH4, and N2O were determined by gas chromatography (APHA, 1989). Redox potential was measured in situ with platinum-tipped electrodes (Karathanasis et al., 2003).
Results and Discussion
Porewater samples showed striking concentration gradients as a function of substrate depth and SAPS cell. A gradual increase of pH with depth and progressive SAPS sequence was evident in the system, ranging from 2.7 to 4.0 in the upper sediment column and from 3.7 to 5.7 at 30 cm depth. More than 99% of total Fe was in the Fe2+ form, with upper limits ranging from about 2,000 mg/L in cell 1 to 400 mg/L in cell 4. In the first three cells Fe increased gradually with depth, showing maximum concentrations at around 20-25 cm, thereafter declining slightly. In the 4th cell maximun Fe concentrations were observed within the upper 5 cm of the sediment column before declining in lower depths. Soluble Fe levels were lowest in the upper 10 cm of the substrate, indicating active precipitation from solution, which is corroborated by the lower pH of the upper sediment column. In contrast, the extremely high Fe concentrations in lower depths suggest that the anaerobic conditions inhibit Fe precipitation. Redox potential measurements indicated that the upper sediment column was within the Eh range favoring Fe oxidation (150-260 mV), while lower substrate depths were maintained in the reduced Fe range (50 to –100 mV). Sulfate followed similar concentration gradients and distribution patterns with Fe, corroborating their close association, particularly in anaerobic substrate zones. The extremely low sulfate values in the upper 5 cm of the sediment column are indicative of the prevailing Fe oxidation and hydrolysis processes and the potential precipitation of Fe-hydroxysulfate minerals. The elevated sulfate levels in aerobic upper sediment column zones of cells 1 and 2 may indicate transformation of Fe-hydroxysulfate minerals to goethite with subsequent release of sulfate ions. Sulfide concentrations were negligible throughout the wetland. Aluminum levels were generally low (2-30 mg/L) and slightly higher in the upper sediment column, where the presence of organic complexes may have enhanced its solubility. Manganese increased gradually with depth from 2 to 28 mg/L, showing the highest concentrations in the first SAPS cell. Considering the high Fe2+ levels and the low pH of the porewaters, it is unlikely that any Mn precipitation may be occurring within the wetland and any potential attenuation should be attributed to sorption on the substrate. Calcium concentrations were maintained relatively constant with depth in the range of 200-300 mg/L, suggesting that the dissolution of the limestone gravel substrate continues to provide its potential maximum buffering capacity.
Conclusions
The findings of this study document the dynamic nature of the geochemistry controlling attenuation and transformation processes within a coal-mine wetland substrate. Depending on slight shifts in pH and Eh, aerobic and anaerobic zones within a wetland substrate may act as pools or sinks of metals, thus causing considerable variability in treatment efficiency. Therefore, treatment design and influent/effluent budget estimates cannot predict wetland performance or guarantee treatment efficiency without a better understanding of spatial geochemical gradients operating within the wetland.
Biogeochemical Processes to Assess the Water Quality Improvement Function of Mitigated Wetlands.
S.M. Wehr-McChesney, E.M. D’ Angelo, and A.D. Karathanasis.
A key function of wetlands is protection of down-gradient surface water bodies from excess nutrient and sediment loading. Biogeochemical carbon, nitrogen, and phosphorus cycling processes in soils largely determine the water quality improvement function of wetlands. At the federal and state levels, there has been increasing interest in restoring and recreating (mitigating) wetlands that have been lost by agricultural, coal mining, and other activities. One of the main unknowns is whether the mitigated wetlands function at the same level as the reference standard wetlands that they are supposed to replace. Several rapid functional assessment methods, including the Hydrogeomorphic (HGM) Method, have been developed to assess how well these systems function compared to their natural counterparts. These methods, however, primarily consist of field observations that are dissociated from the biogeochemical processes that they are designed to represent. The objectives of this research were to: (i) compare N and C cycling processes in bottomland hardwood wetlands and mitigated wetlands at various successional stages, and (ii) relate biogeochemical cycling processes to parameters measured in the HGM Method for assessing wetland functions. Seventeen wetland sites of various ages (0 to >30 yr) and reference standard sites of the bottomland hardwood class in western Kentucky were selected with the cooperation of the Army Corp of Engineers. Litter and soil samples (0-5cm depth, 5-20 cm depth) were collected in the summer and spring 2001-2002 and incubated in laboratory microcosms for the determination of biogeochemical processes including carbon and nitrogen storage, mineralization, denitrification, nitrification and inorganic N fluxes between the flooded soil-water column. In the summer of 2002, field measurements were taken in accordance with the HGM Method to assess the functions related to water quality improvement. Results showed major differences among biogeochemical cycling processes as a function of wetland mitigation stage. In spite of some statistical inconsistency in the quantitative relationships between biogeochemical cycling rates and HGM functional capacity indices, there were distinct trends among the wetland groups surveyed. These findings demonstrated that soil and litter biogeochemical properties can be used as supplemental or refinement tools along with the current HGM approach to assess or predict water quality improvement functions in restored wetland ecosystems.
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