Impact of biochar on plant growth and uptake of ciprofloxacin, triclocarban and triclosan from biosolids.

Application of municipal biosolids in agriculture present a concern with potential uptake and bioaccumulation of pharmaceutical compounds from biosolids into agronomic plants. We evaluated the efficacy of biochar as a soil amendment to minimize uptake of antimicrobial agents (ciprofloxacin, triclocarban, and triclosan) in lettuce (Lactuca sativa) and carrot (Daucus carota) plants. Biochar reduced the concentration of ciprofloxacin and triclocarban in lettuce leaves and resulted in a 67% reduction of triclosan in carrot roots. There was no substantial difference in pharmaceutical concentrations in carrot and lettuce plant matter at low (2.0 g kg-1 soil) and high (20.4 g kg-1 soil) rates of applied biochar. The co-amendment of biochar and biosolids increased soil pH and nutrient content which were positively correlated with an increase in lettuce shoot biomass. Our results demonstrate the potential efficacy of using walnut shell biochar as a sorbent for pharmaceutical contaminants in soil without negatively affecting plant growth.

Impact of Irrigation Strategies on Tomato Root Distribution and Rhizosphere Processes in an Organic System.

Root exploitation of soil heterogeneity and microbially mediated rhizosphere nutrient transformations play critical roles in plant resource uptake. However, how these processes change under water-saving irrigation technologies remains unclear, especially for organic systems where crops rely on soil ecological processes for plant nutrition and productivity. We conducted a field experiment and examined how water-saving subsurface drip irrigation (SDI) and concentrated organic fertilizer application altered root traits and rhizosphere processes compared to traditional furrow irrigation (FI) in an organic tomato system. We measured root distribution and morphology, the activities of C-, N-, and P-cycling enzymes in the rhizosphere, the abundance of rhizosphere microbial N-cycling genes, and root mycorrhizal colonization rate under two irrigation strategies. Tomato plants produced shorter and finer root systems with higher densities of roots around the drip line, lower activities of soil C-degrading enzymes, and shifts in the abundance of microbial N-cycling genes and mycorrhizal colonization rates in the rhizosphere of SDI plants compared to FI. SDI led to 66.4% higher irrigation water productivity than FI, but it also led to excessive vegetative growth and 28.3% lower tomato yield than FI. Our results suggest that roots and root-microbe interactions have a high potential for coordinated adaptation to water and nutrient spatial patterns to facilitate resource uptake under SDI. However, mismatches between plant needs and resource availability remain, highlighting the importance of assessing temporal dynamics of root-soil-microbe interactions to maximize their resource-mining potential for innovative irrigation systems.

Yields and resilience outcomes of organic, cover crop, and conventional practices in a Mediterranean climate.

Adaptive management practices that maximize yields while improving yield resilience are required in the face of resource variability and climate change. Ecological intensification such as organic farming and cover cropping are lauded in some studies for fostering yield resilience, but subject to criticism in others for their low productivity. We implemented a quantitative framework to assess yield resilience, emphasizing four aspects of yield dynamics: yield, yield stability, yield resistance (i.e., the ability of systems to avoid crop failure under stressful growing conditions), and maximum yield potential. We compared the resilience of maize-tomato rotation systems after 24 years of irrigated organic, cover cropped, and conventional management in a Mediterranean climate, and identified crop-specific resilience responses of tomato and maize to three management systems. Organic management maintained tomato yields comparable to those under conventional management, while increasing yield stability and resistance. However, organic and cover cropped system resulted in 36.1% and 35.8% lower maize yields and reduced yield stability and resistance than the conventional system. Our analyses suggest that investments in ecological intensification approaches could potentially contribute to long-term yield resilience, however, these approaches need to be tailored for individual crops and systems to maximize their benefits, rather than employing one-size-fits-all approaches.

Deep soil inventories reveal that impacts of cover crops and compost on soil carbon sequestration differ in surface and subsurface soils.

Increasing soil organic carbon (SOC) via organic inputs is a key strategy for increasing long-term soil C storage and improving the climate change mitigation and adaptation potential of agricultural systems. A long-term trial in California's Mediterranean climate revealed impacts of management on SOC in maize-tomato and wheat-fallow cropping systems. SOC was measured at the initiation of the experiment and at year 19, at five depth increments down to 2 m, taking into account changes in bulk density. Across the entire 2 m profile, SOC in the wheat-fallow systems did not change with the addition of N fertilizer, winter cover crops (WCC), or irrigation alone and decreased by 5.6% with no inputs. There was some evidence of soil C gains at depth with both N fertilizer and irrigation, though high variation precluded detection of significant changes. In maize-tomato rotations, SOC increased by 12.6% (21.8 Mg C/ha) with both WCC and composted poultry manure inputs, across the 2 m profile. The addition of WCC to a conventionally managed system increased SOC stocks by 3.5% (1.44 Mg C/ha) in the 0-30 cm layer, but decreased by 10.8% (14.86 Mg C/ha) in the 30-200 cm layer, resulting in overall losses of 13.4 Mg C/ha. If we only measured soil C in the top 30 cm, we would have assumed an increase in total soil C increased with WCC alone, whereas in reality significant losses in SOC occurred when considering the 2 m soil profile. Ignoring the subsoil carbon dynamics in deeper layers of soil fails to recognize potential opportunities for soil C sequestration, and may lead to false conclusions about the impact of management practices on C sequestration.

The century experiment: the first twenty years of UC Davis' Mediterranean agroecological experiment.

The Century Experiment at the Russell Ranch Sustainable Agriculture Facility at the University of California, Davis provides long-term agroecological data from row crop systems in California's Central Valley starting in 1993. The Century Experiment was initially designed to study the effects of a gradient of water and nitrogen availability on soil properties and crop performance in ten different cropping systems to measure tradeoffs and synergies between agricultural productivity and sustainability. Currently systems include 11 different cropping systems-consisting of four different crops and a cover crop mixture-and one native grass system. This paper describes the long-term core data from the Century Experiment from 1993-2014, including crop yields and biomass, crop elemental contents, aerial-photo-based Normalized Difference Vegetation Index data, soil properties, weather, chemical constituents in irrigation water, winter weed populations, and operational data including fertilizer and pesticide application amounts and dates, planting dates, planting quantity and crop variety, and harvest dates. This data set represents the only known long-term set of data characterizing food production and sustainability in irrigated and rainfed Mediterranean annual cropping systems. There are no copyright restrictions associated with the use of this dataset.

Use of nuclear receptor luciferase-based bioassays to detect endocrine active chemicals in a biosolids-biochar amended soil.

Biosolids are a potentially valuable source of carbon and nutrients for agricultural soils; however, potential unintended impacts on human health and the environment must be considered. Virtually all biosolids contain trace amounts endocrine-disrupting chemicals derived from human use of pharmaceuticals and personal care products (PPCPs). One potential way to reduce the bioavailability of PPCPs is to co-apply biosolids with biochar to soil, because biochar's chemical (e.g., aromaticity) and physical properties (e.g., surface area) give it a high affinity to bind many organic chemicals in the environment. We developed a soil-specific extraction method and utilized a luciferase-based bioassay (CALUX) to detect endocrine active chemicals in a biosolids-biochar co-amendment soil greenhouse study. Both biochar (walnut shell, 900 °C) and biosolids had positive impacts on carrot and lettuce biomass accumulation over our study period. However, the walnut shell biochar stimulated aryl hydrocarbon receptor activity, suggesting the presence of potential endocrine active chemicals in the biochar. Since the biochar rate tested (100 t ha-1) is above the average agronomic rate (10-20 t ha-1), endocrine effects would not be expected in most environmental applications. The effect of high temperature biochars on endocrine system pathways must be explored further, using both quantitative analytical tools to identify potential endocrine active chemicals and highly sensitive bioanalytical assays such as CALUX to measure the resulting biological activity of such compounds.

Effect of benzene and ethylbenzene on the transcription of methyl-tert-butyl ether degradation genes of Methylibium petroleiphilum PM1.

Methyl-tert-butyl ether (MTBE) and its degradation by-product, tert-butyl alcohol (TBA), are widespread contaminants detected frequently in groundwater in California. Since MTBE was used as a fuel oxygenate for almost two decades, leaking underground fuel storage tanks are an important source of contamination. Gasoline components such as BTEX (benzene, toluene, ethylbenzene and xylenes) are often present in mixtures with MTBE and TBA. Investigations of interactions between BTEX and MTBE degradation have not yielded consistent trends, and the molecular mechanisms of BTEX compounds' impact on MTBE degradation are not well understood. We investigated trends in transcription of biodegradation genes in the MTBE-degrading bacterium, Methylibium petroleiphilum PM1 upon exposure to MTBE, TBA, ethylbenzene and benzene as individual compounds or in mixtures. We designed real-time quantitative PCR assays to target functional genes of strain PM1 and provide evidence for induction of genes mdpA (MTBE monooxygenase), mdpJ (TBA hydroxylase) and bmoA (benzene monooxygenase) in response to MTBE, TBA and benzene, respectively. Delayed induction of mdpA and mdpJ transcription occurred with mixtures of benzene and MTBE or TBA, respectively. bmoA transcription was similar in the presence of MTBE or TBA with benzene as in their absence. Our results also indicate that ethylbenzene, previously proposed as an inhibitor of MTBE degradation in some bacteria, inhibits transcription of mdpA, mdpJ and bmoAgenes in strain PM1.

Impact of biochar amendment on soil water soluble carbon in the context of extreme hydrological events.

Biochar amendments to soil have been promoted as a low cost carbon (C) sequestration strategy as well as a way to increase nutrient retention and remediate contaminants. If biochar is to become part of a long-term management strategy, it is important to consider its positive and negative impacts, and their trade-offs, on soil organic matter (SOM) and soluble C under different hydrological conditions such as prolonged drought or frequent wet-dry cycles. A 52-week incubation experiment measuring the influence of biochar on soil water soluble C under different soil moisture conditions (wet, dry, or wet-dry cycles) indicated that, in general, dry and wet-dry cycles increased water soluble C, and biochar addition further increased release of water soluble C from native SOM. Biochar amendment appeared to increase transformation of native SOM to water soluble C, based on specific ultraviolet absorption (SUVA) and C stable isotope composition; however, the increased amount of water soluble C from native SOM is less than 1% of total biochar C. The impacts of biochar on water soluble C need to be carefully considered when applying biochar to agricultural soil.

Phenylurea herbicide sorption to biochars and agricultural soil.

Biochar is increasingly been used as a soil amendment to improve water-holding capacity, reduce nutrient leaching, increase soil pH, and also as a means to reduce contamination through sorption of heavy metals or organic pollutants. The sorption behavior of three phenylurea herbicides (monuron, diuron and linuron) on five biochars (Enhanced Biochar, Hog Waste, Turkey Litter, Walnut Shell and Wood Feedstock) and an agricultural soil (Yolo silt loam) was investigated using a batch equilibration method. Sorption isotherms of herbicides to biochars were well described by the Freundlich model (R(2) = 0.93-0.97). The adsorption KF values ranged from 6.94 to 1306.95 mg kg(-1) and indicated the sorption of herbicides in the biochars and Yolo soil was in the sequence of linuron > diuron > monuron and walnut shell biochar > wood feedstock biochar > turkey litter biochar > enhanced biochar > hog waste biochar > Yolo soil. These data show that sorption of herbicides to biochar can have both positive (reduced off-site transport) and negative (reduced herbicide efficacy) implications and specific biochar properties, such as H/C ratio and surface area, should be considered together with soil type, agriculture chemical and climate condition in biochar application to agricultural soil to optimize the system for both agricultural and environmental benefits.

Shamba Maisha: Pilot agricultural intervention for food security and HIV health outcomes in Kenya: design, methods, baseline results and process evaluation of a cluster-randomized controlled trial.

BACKGROUND: Despite advances in treatment of people living with HIV, morbidity and mortality remains unacceptably high in sub-Saharan Africa, largely due to parallel epidemics of poverty and food insecurity. METHODS/DESIGN: We conducted a pilot cluster randomized controlled trial (RCT) of a multisectoral agricultural and microfinance intervention (entitled Shamba Maisha) designed to improve food security, household wealth, HIV clinical outcomes and women's empowerment. The intervention was carried out at two HIV clinics in Kenya, one randomized to the intervention arm and one to the control arm. HIV-infected patients >18 years, on antiretroviral therapy, with moderate/severe food insecurity and/or body mass index (BMI) <18.5, and access to land and surface water were eligible for enrollment. The intervention included: 1) a microfinance loan (~$150) to purchase the farming commodities, 2) a micro-irrigation pump, seeds, and fertilizer, and 3) trainings in sustainable agricultural practices and financial literacy. Enrollment of 140 participants took four months, and the screening-to-enrollment ratio was similar between arms. We followed participants for 12 months and conducted structured questionnaires. We also conducted a process evaluation with participants and stakeholders 3-5 months after study start and at study end. DISCUSSION: Baseline results revealed that participants at the two sites were similar in age, gender and marital status. A greater proportion of participants at the intervention site had a low BMI in comparison to participants at the control site (18% vs. 7%, p = 0.054). While median CD4 count was similar between arms, a greater proportion of participants enrolled at the intervention arm had a detectable HIV viral load compared with control participants (49% vs. 28%, respectively, p < 0.010). Process evaluation findings suggested that Shamba Maisha had high acceptability in recruitment, delivered strong agricultural and financial training, and led to labor saving due to use of the water pump. Implementation challenges included participant concerns about repaying loans, agricultural challenges due to weather patterns, and a challenging partnership with the microfinance institution. We expect the results from this pilot study to provide useful data on the impacts of livelihood interventions and will help in the design of a definitive cluster RCT. TRIAL REGISTRATION: This trial is registered at ClinicalTrials.gov, NCT01548599.

Gene mdpC plays a regulatory role in the methyl-tert-butyl ether degradation pathway of Methylibium petroleiphilum strain PM1.

Among the few bacteria known to utilize methyl tert-butyl ether (MTBE) as a sole carbon source, Methylibium petroleiphilum PM1 is a well-characterized organism with a sequenced genome; however, knowledge of the genetic regulation of its MTBE degradation pathway is limited. We investigated the role of a putative transcriptional activator gene, mdpC, in the induction of MTBE-degradation genes mdpA (encoding MTBE monooxygenase) and mdpJ (encoding tert-butyl alcohol hydroxylase) of strain PM1 in a gene-knockout mutant mdpC(-). We also utilized quantitative reverse transcriptase PCR assays targeting genes mdpA, mdpJ and mdpC to determine the effects of the mutation on transcription of these genes. Our results indicate that gene mdpC is involved in the induction of both mdpA and mdpJ in response to MTBE and tert-butyl alcohol (TBA) exposure in PM1. An additional independent mechanism may be involved in the induction of mdpJ in the presence of TBA.

Successful treatment of an MTBE-impacted aquifer using a bioreactor self-colonized by native aquifer bacteria.

A field-scale fixed bed bioreactor was used to successfully treat an MTBE-contaminated aquifer in North Hollywood, CA without requiring inoculation with introduced bacteria. Native bacteria from the MTBE-impacted aquifer rapidly colonized the bioreactor, entering the bioreactor in the contaminated groundwater pumped from the site, and biodegraded MTBE with greater than 99 % removal efficiency. DNA sequencing of the 16S rRNA gene identified MTBE-degrading bacteria Methylibium petroleiphilum in the bioreactor. Quantitative PCR showed M. petroleiphilum enriched by three orders of magnitude in the bioreactor above densities pre-existing in the groundwater. Because treatment was carried out by indigenous rather than introduced organisms, regulatory approval was obtained for implementation of a full-scale bioreactor to continue treatment of the aquifer. In addition, after confirmation of MTBE removal in the bioreactor to below maximum contaminant limit levels (MCL; MTBE = 5 µg L(-1)), treated water was approved for reinjection back into the aquifer rather than requiring discharge to a water treatment system. This is the first treatment system in California to be approved for reinjection of biologically treated effluent into a drinking water aquifer. This study demonstrated the potential for using native microbial communities already present in the aquifer as an inoculum for ex-situ bioreactors, circumventing the need to establish non-native, non-acclimated and potentially costly inoculants. Understanding and harnessing the metabolic potential of native organisms circumvents some of the issues associated with introducing non-native organisms into drinking water aquifers, and can provide a low-cost and efficient remediation technology that can streamline future bioremediation approval processes.

Effect of different transport observations on inverse modeling results: case study of a long-term groundwater tracer test monitored at high resolution.

Conservative tracer experiments can provide information useful for characterizing various subsurface transport properties. This study examines the effectiveness of three different types of transport observations for sensitivity analysis and parameter estimation of a three-dimensional site-specific groundwater flow and transport model: conservative tracer breakthrough curves (BTCs), first temporal moments of BTCs (m1), and tracer cumulative mass discharge (Md) through control planes combined with hydraulic head observations (h). High-resolution data obtained from a 410-day controlled field experiment at Vandenberg Air Force Base, California (USA), have been used. In this experiment, bromide was injected to create two adjacent plumes monitored at six different transects (perpendicular to groundwater flow) with a total of 162 monitoring wells. A total of 133 different observations of transient hydraulic head, 1,158 of BTC concentration, 23 of first moment, and 36 of mass discharge were used for sensitivity analysis and parameter estimation of nine flow and transport parameters. The importance of each group of transport observations in estimating these parameters was evaluated using sensitivity analysis, and five out of nine parameters were calibrated against these data. Results showed the advantages of using temporal moment of conservative tracer BTCs and mass discharge as observations for inverse modeling.

Impacts of an ethanol-blended fuel release on groundwater and fate of produced methane: Simulation of field observations.

In a field experiment at Vandenberg Air Force Base (VAFB) designed to mimic the impact of a small-volume release of E10 (10% ethanol and 90% conventional gasoline), two plumes were created by injecting extracted groundwater spiked with benzene, toluene, and o-xylene, abbreviated BToX (No-Ethanol Lane) and BToX plus ethanol (With-Ethanol Lane) for 283 days. We developed a reactive transport model to understand processes controlling the fate of ethanol and BToX. The model was calibrated to the extensive field dataset and accounted for concentrations of sulfate, iron, acetate, and methane along with iron-reducing bacteria, sulfate-reducing bacteria, fermentative bacteria, and methanogenic archaea. The benzene plume was about 4.5 times longer in the With-Ethanol Lane than in the No-Ethanol Lane. Matching this different behavior in the two lanes required inhibiting benzene degradation in the presence of ethanol. Inclusion of iron reduction with negligible growth of iron-reducers was required to reproduce the observed constant degradation rate of benzene. Modeling suggested that vertical dispersion and diffusion of sulfate from an adjacent aquitard were important sources of sulfate in the aquifer. Matching of methane data required incorporating initial fermentation of ethanol to acetate, methane loss by outgassing, and methane oxidation coupled to sulfate and iron reduction. Simulation of microbial growth using dual Monod kinetics, and including inhibition by more favorable electron acceptors, generally resulted in reasonable yields for microbial growth of 0.01-0.05.

Effects of triclosan and biosolids on microbial community composition in an agricultural soil.

Triclosan (TCS) is a widely used antimicrobial agent found at high concentrations in biosolids produced during municipal wastewater treatment. The effect of adding TCS, in the presence or absence of biosolids, on the composition of an agricultural soil microbial community was measured using phospholipid fatty acid analysis (PLFA). Most changes observed in microbial community composition were attributable to the addition of biosolids or to the passage of time, with smaller changes due to TCS exposure, regardless of the presence of biosolids. TCS slightly reduced the relative abundance of Gram-positive and Gram-negative bacteria and fungi, with or without biosolids. Bacteria were more sensitive than eukaryotes, consistent with the mode of action of TCS, which selectively targets fatty acid synthesis and disrupts cell membranes of bacteria. TCS slightly increased biomarkers of microbial stress, but stress biomarkers were lower in all biosolid treated soils, presumably due to increased availability of nutrients mitigating potential TCS toxicity.

Using DNA-Stable Isotope Probing to Identify MTBE- and TBA-Degrading Microorganisms in Contaminated Groundwater.

Although the anaerobic biodegradation of methyl tert-butyl ether (MTBE) and tert-butyl alcohol (TBA) has been documented in the laboratory and the field, knowledge of the microorganisms and mechanisms involved is still lacking. In this study, DNA-stable isotope probing (SIP) was used to identify microorganisms involved in anaerobic fuel oxygenate biodegradation in a sulfate-reducing MTBE and TBA plume. Microorganisms were collected in the field using Bio-Sep® beads amended with 13C5-MTBE, 13C1-MTBE (only methoxy carbon labeled), or13C4-TBA. 13C-DNA and 12C-DNA extracted from the Bio-Sep beads were cloned and 16S rRNA gene sequences were used to identify the indigenous microorganisms involved in degrading the methoxy group of MTBE and the tert-butyl group of MTBE and TBA. Results indicated that microorganisms were actively degrading 13C-labeled MTBE and TBA in situ and the 13C was incorporated into their DNA. Several sequences related to known MTBE- and TBA-degraders in the Burkholderiales and the Sphingomonadales orders were detected in all three13C clone libraries and were likely to be primary degraders at the site. Sequences related to sulfate-reducing bacteria and iron-reducers, such as Geobacter and Geothrix, were only detected in the clone libraries where MTBE and TBA were fully labeled with 13C, suggesting that they were involved in processing carbon from the tert-butyl group. Sequences similar to the Pseudomonas genus predominated in the clone library where only the methoxy carbon of MTBE was labeled with 13C. It is likely that members of this genus were secondary degraders cross-feeding on 13C-labeled metabolites such as acetate.

Primers for amplification of nitrous oxide reductase genes associated with Firmicutes and Bacteroidetes in organic-compound-rich soils.

The nosZ gene encodes nitrous oxide reductase, a key enzyme in the nitrous oxide reduction that occurs during complete denitrification. Many conventional approaches have used Proteobacteria-based primers to detect nosZ in environmental samples. However, these primers often fail to detect nosZ in non-Proteobacteria strains, including Firmicutes (Gram-positive) and Bacteroidetes. In this study, newly designed nosZ primers successfully amplified this gene from five Geobacillus species (Firmicutes). The primers were used to construct nosZ clone libraries from DNA extracted from sludge and domestic animal feedlot soils, all with high organic carbon contents. After DNA sequencing, phylogenetic analysis identified many new nosZ sequences with high levels of homology to nosZ from Bacteroidetes, probably because of the high sequence similarity of nosZ from Firmicutes and Bacteroidetes, and a predominance of Bacteroidetes in feedlot environments. Three sets of new quantitative real-time PCR (qPCR) primers based on our clone library sequences were designed and tested for their specificities. Our data showed that only Bacteroidetes-related nosZ sequences were amplified, whereas conventional Proteobacteria-based primers amplified only Proteobacteria-related nosZ. Quantitative analysis of nosZ with the new qPCR primers recovered ~10(4) copies per 100 ng DNA. Thus, it appears that amplification with conventional primers is insufficient for developing an understanding of the diversity and abundance of nosZ genes in the environment.

An ex situ evaluation of TBA- and MTBE-baited bio-traps.

Aquifer microbial communities can be investigated using Bio-traps(®) ("bio-traps"), passive samplers containing Bio-Sep(®) beads ("bio-beads") that are deployed in monitoring wells to be colonized by bacteria delivered via groundwater flow through the well. When bio-beads are "baited" with organic contaminants enriched in (13)C, stable isotope probing allows assessment of the composition and activity of the microbial community. This study used an ex situ system fed by groundwater continuously extracted from an adjacent monitoring well within an experimentally-created aerobic zone treating a tert-butyl alcohol (TBA) plume. The goal was to evaluate aspects of bio-trap performance that cannot be studied quantitatively in situ. The measured groundwater flow through a bio-trap housing suggests that such traps might typically "sample" about 1.8 L per month. The desorption of TBA or methyl tert-butyl ether (MTBE) bait from bio-traps during a typical deployment duration of 6 weeks was approximately 90% and 45%, respectively, of the total initial bait load, with initially high rate of mass loss that decreased markedly after a few days. The concentration of TBA in groundwater flowing by the TBA-baited bio-beads was estimated to be as high as 3400 mg/L during the first few days, which would be expected to inhibit growth of TBA-degrading microbes. Initial inhibition was also implied for the MTBE-baited bio-trap, but at lower concentrations and for a shorter time. After a few days, concentrations in groundwater flowing through the bio-traps dropped below inhibitory concentrations but remained 4-5 orders of magnitude higher than TBA or MTBE concentrations within the aquifer at the experimental site. Desorption from the bio-beads during ex situ deployment occurred at first as predicted by prior sorption analyses of bio-beads but with apparent hysteresis thereafter, possibly due to mass transfer limitations caused by colonizing microbes. These results suggest that TBA- or MTBE-baited bio-traps could be baited at lower initial total mass loading with no detriment to trapping ability. The bio-traps were able to collect detectable amounts of microbial DNA and thus allow some insight into the sparse microbial community present in the aquifer during remediation of the low concentration plume.

Mass discharge in a tracer plume: evaluation of the Theissen Polygon Method.

A tracer plume was created within a thin aquifer by injection for 299 d of two adjacent "sub-plumes" to represent one type of plume heterogeneity encountered in practice. The plume was monitored by snapshot sampling of transects of fully screened wells. The mass injection rate and total mass injected were known. Using all wells in each transect (0.77 m well spacing, 1.4 points/m(2) sampling density), the Theissen Polygon Method (TPM) yielded apparently accurate mass discharge (M(d) ) estimates at three transects for 12 snapshots. When applied to hypothetical sparser transects using subsets of the wells with average spacing and sampling density from 1.55 to 5.39 m and 0.70 to 0.20 points/m(2) , respectively, the TPM accuracy depended on well spacing and location of the wells in the hypothesized transect with respect to the sub-plumes. Potential error was relatively low when the well spacing was less than the widths of the sub-plumes (>0.35 points/m(2) ). Potential error increased for well spacing similar to or greater than the sub-plume widths, or when less than 1% of the plume area was sampled. For low density sampling of laterally heterogeneous plumes, small changes in groundwater flow direction can lead to wide fluctuations in M(d) estimates by the TPM. However, sampling conducted when flow is known or likely to be in a preferred direction can potentially allow more useful comparisons of M(d) over multiyear time frames, such as required for performance evaluation of natural attenuation or engineered remediation systems.