Fluctuating Water Table Conditions Research Articles

Dynamics of microbial populations and diversity in NAPL contaminated peat soil under varying water table conditions

Despite the risks that hydrocarbon contamination from pipeline leaks or train derailments impose on the health of peatlands in hydrocarbon production areas and transportation corridors, assessing the effect of such contaminations on the health and sustainability of peatlands has received little attention. This study investigates the impacts of hydrocarbons on peat microbial communities. Column experiments were conducted on non-aqueous phase liquid (NAPL) contaminated undisturbed peat core (0–35 cm) under static and fluctuating water table conditions. Water table fluctuations reduced residual NAPL saturation from 8.1-11.3% to 7.7–9.5%. Biodegradation of n-C8 and n-C12 along with oxidation of CH4 together produced high CO2 concentrations in the headspace. Clear patterns in dynamics in the microbial community structure were observed, with a more pronounced population growth. However, a significant loss of microbial richness was observed in contaminated columns. The result indicates that the phylum Proteobacteria benefited most from NAPL; however, their families differed between static and fluctuating water table conditions. This study established strong evidence that peat microbes and water table fluctuation can be an excellent tool for hydrocarbon removal and its control in peatlands.

Water use efficiency and yield of cowpea and nutrient loss in lysimeter experiment under varying water table depth, irrigation schedules and irrigation method

Lysimeter experiment was conducted at Govind Ballabh Pant University of Agriculture & Technology, Pantnagar during summer season 2013 to study the effect of irrigation schedules and methods on yield, nutrient uptake and water use efficiency of cowpea as well as nutrient loss from silty clay loam soil under fluctuating water table conditions. The experiment was laid out in factorial randomized block design having three irrigation schedules at IW/CPE ratio of 0.3. 0.2 and 0.15 with two irrigation methods (flood and sprinkler) and at 30±1.5, 60±1.5 and 90±1.5 cm water tables replicated thrice. Maximum root length (129.4 cm) and root length density (0.395 cm/cm3) were obtained when irrigation was scheduled at IW: CPE 0.3 associated with 30±1.5 cm water table depth using sprinkler method. Increase in water table depth and IW: CPE ratio decreased water use efficiency where IW: CPE 0.3 produced highest grain yield (1411.6 kg ha-1) with the WUE of 1.15 kg ha mm-1. Significant nutrients uptake response was observed owing to variation in water table depth, irrigation schedules and methods. Analysis of lysimeter leached water showed that with deep drainage and more IW:CPE, leaching losses of N,P and K were more however water applied through sprinkler saved 20.1, 53.7 and 24.4% N, P and K, respectively, over flooded method. Irrigation given at IW: CPE 0.3 through sprinkler form at 60±1.5 cm water table depth favours the higher grain yield and nutrient uptake by crop whereas flooded irrigation with deep water table condition accelerated nutrient leaching.SAARC J. Agri., 14(2): 46-55 (2016)

Impact of Water Table Fluctuations on the Concentration of Borehole Gas from NAPL Sources in the Vadose Zone

Core Ideas Field borehole TCE gas concentration responded to water table fluctuations. Laboratory and numerical tests were performed to analyze the response patterns. Borehole gas data can be used for identification of contamination sources. Dense non‐aqueous‐phase liquids (DNAPLs), such as trichloroethylene (TCE), adversely affect groundwater quality because they are difficult to remove from the subsurface environment. Trichloroethylene is volatile, and TCE gas can diffuse into a borehole to change the borehole gas concentration. Thus, borehole TCE gas monitoring can provide information on the presence of a TCE source. In this study, borehole gas extraction tests were conducted at a field site contaminated by TCE and its daughter products. Temporal changes in borehole gas concentrations appeared to be closely related to changes in the level of the water table. Laboratory experiments were performed to demonstrate that changes in gas concentrations were due to water table fluctuation at boreholes located near residual TCE point sources. Three source configurations for the borehole TCE gas migration, generated from either DNAPL, TCE, or dissolved TCE, were used to design two‐dimensional sand tank experiments. Under fluctuating water table conditions, DNAPL TCE sources in the unsaturated zone produced slow response times but high and long‐lasting gas concentrations. In contrast, dissolved TCE sources produced relatively weak gas concentrations characterized by a peak, quickly followed by decreasing gas concentration curves. The Subsurface Transport Over Multiple Phases (STOMP) simulator was used to simulate breakthrough curves under fluctuating water table conditions. The results of the sand tank experiments and numerical simulations both suggest that a fluctuating water table is a significant factor in determining borehole gas concentrations. The results also show that the combination of borehole gas and groundwater monitoring has an important role in field applications because the changing pattern of borehole gas concentrations can be used to understand characteristics of the presence of DNAPL sources, whether these remained only in the unsaturated zone or were also dissolved in groundwater.

Factors related to Bh-horizon depth for artificial and natural E–Bh sequences

Aquods occur extensively in the coastal plain of the SE USA, where podzolization is mainly restricted to fluctuating water table conditions. Depths to upper boundaries of Spodic (Bh) horizons vary from a few cm to ≥75cm even under similar drainage conditions. Hence, seasonal water table depth parameters do not explain the wide variation in boundary depth. One objective of this study was to experimentally test the following hypotheses: (H1) Depth to the E–Bh boundary relates inversely to concentration of metal oxides, silt, and clay content of initial (“parent”) material for similar hydrologic conditions and organic acid activity; (H2) depth to that boundary is greater under higher organic activity for a given soil material and hydrologic condition. Another objective was to statistically evaluate how the composition of >270 natural Spodic horizons collected during the Florida Cooperative Soil Survey Program related to upper boundary depth. Thirteen sandy coastal plain soil materials of varying noncrystalline-metal-oxide- (Al and Fe), silt-, and clay contents were packed in columns and subjected to daily cycles of fluctuating water tables and dilute organic acid applications. Artificial E–Bh boundaries of variable depths were generated in the columns. Boundaries tended to be abrupt but topographically irregular. Clay- and extractable-noncrystalline Al and Fe contents were inversely related to final depth of artificial Bh horizons for the same water-table conditions. Higher oxalic acid concentrations produced deeper boundaries in the same materials; hence, local biogeochemical factors related to organic acid production are also likely to influence the ultimate Aquod morphology on the landscape. Silt-, clay-, and metal-oxide contents were significantly higher in shallower than in deeper natural Florida Spodic horizons.

Fluctuating water table effect on phosphorus release and availability from a Florida Spodosol

Spodosols in Florida exhibit a unique hydrology including a fluctuating water table that can often reach the surface horizon during the summer months. This paper evaluated the effects of fluctuating water table on P fluxes and availability in a typical Florida Spodosol. The study was conducted on an established bahiagrass (Paspalum notatum Flugge) pasture grown on a Smyrna sand (sandy, siliceous, hyperthermic Aeric Alaquods). Phosphorus fluxes were measured using suction cup lysimeters installed at depths of 15, 30, 60, 90, and 150 cm. The 15- and 30-cm deep lysimeters were located above the spodic (Bh) horizon, whereas the remaining lysimeters (60-, 90- and 150-cm) were below the Bh horizon. A pressure transducer was installed at the center of the experimental site to monitor changes in water table depth. Two anion exchange membranes (2 × 6 cm) were buried in each plot at a 15-cm depth to estimate in situ P availability. During the 2-year study, leachate P concentrations in the lysimeters above the Bh horizon increased as water tables rose in the months of August and September. Conversely, P concentration measured in the lysimeters below the Bh horizon remained relatively constant (0.02 mg L−1). Soil P availability also increased (from 3.2 μg cm−2 in June to 9.2 μg cm−2 in August) in response to rising water table. Results showed that the fluctuating water table conditions experienced during the summer months in Florida cause upward flux of P from the Bh horizon, which increased soil P availability and susceptibility to off-site transport.

A Controlled Experiment to Determine the Water Table Response Using Ground Penetrating Radar

Ground penetrating radar (GPR) data were explored during a physical model experiment that utilized a polyethylene tank and an abiotic sand matrix to simulate water contamination in the vadose zone under fluctuating water table conditions. The main objective of this experiment was to determine the source of GPR reflections from a changing water table in a controlled experimental environment. During the experiment, the water table was varied by injecting water into the bottom of the tank and subsequently draining it. The experiment was conducted over a number of days, during which various conditions (unsaturated, saturated, residual saturation) were characterized using GPR measurements. As a result of this approach, a more comprehensive comparison was possible for each step and results were compared with mass balance information. Results of the study indicate that, under saturated conditions, the main reflector of GPR energy is indicative of the capillary fringe and not the actual water table. Well readings and estimates of the mass balance of water input into the system confirm this interpretation.

Changes across Artificial E–Bh Boundaries Formed under Simulated Fluctuating Water Tables

Spodosols in the coastal plain of the southeastern USA are mainly restricted to zones of fluctuating water table, but mechanisms of water table linkage are not well established. A previous study showed that Aquod‐like E and Bh horizons could be artificially formed only under simulated fluctuating water table conditions. The present study was conducted to characterize the redistribution of components during artificial podzolization and to compare the resultant distributions with those of an Aquod adjacent to the Psamment from which column materials were collected. A hypothesis tested is that the E–Bh formation involves colloidal as well as chemical translocation. Artificial E–Bh horizons were generated in three acrylic columns using an approximate cycle of 22 h of saturation and 2 h of free drainage. Distilled water was introduced from the bottom of columns, and oxalic acid from the top. Boundaries migrated downward at a progressively slower rate, and became increasingly abrupt and irregular with time. E horizons consisted mainly of stripped sand grains; coatings were retained in Bh horizons. Fine materials containing C and Al accumulated in the upper centimeters of the Bh, as documented by scanning electron microscopy (SEM) and energy dispersive x‐ray analysis (EDX). Bulk analyses verified accumulations of C in the Bh and a movement of Al and crystalline clay from E to Bh. Artificial E–Bh formation entailed redistribution of sand grain coatings initially present on sands used in columns. Crystalline as well as noncrystalline clay accumulated in the Bh horizon, supporting the idea that E–Bh formation involved colloidal as well as chemical migration. A weatherable‐mineral source of Al is not required for the artificial water table–induced formation of E–Bh sequences. Noncrystalline Al, dissolved in the grain‐stripping process, is a likely source of Al in Bh horizons of the region. Distributional trends in artificial E–Bh sequences paralleled those of the natural Aquod.

WATER TABLE DYNAMICS ACROSS AN AQUOD-UDULT TRANSITION IN FLORIDA FLATWOODS

Spodosol formation in the coastal plain of the southeastern United States is largely restricted to areas where there are fluctuating water table conditions within the zone of podzolization. The purpose of this study was to document specific hydrologic conditions associated with podzolization, and to address the idea that frequency and duration of saturation are critical threshold factors triggering and driving the process regionally. The study was conducted along an abrupt Aquod-to-Udult transition, which showed the commonly observed trend of an upward curving spodic horizon (Bh) that merges with the A horizon of the Udult. Objectives were to (i) compare the Aquod and the Udult with respect to depth and duration of saturation and (ii) determine hydraulic responses of horizons to rainfall events along the Aquod-Udult transition. Water table fluctuations within a rectangular area (15 × 25 m) encompassing the soils were monitored, using piezometers, over a 1-year period. Hydraulic and horizon surfaces were interpolated by Kriging. Results confirmed that the water table sloped sharply downward across the Aquod-to-Udult transition during most of the sampling period but was nearly level for brief periods after heavy rains. The hydraulic response of the Udult argillic horizon to rain events was much slower than that of the Aquod argillic horizon, such that the Udult periodically experienced epiaquic conditions. The hydraulic head of the Aquod argillic horizon was consistently higher than that of the Udult argillic horizon; discharge from the Aquod argillic compensated for evapotranspiration and maintained wetter conditions in the upper sandy horizons. Depth and color development of the Aquod Bh horizon were inversely related to water table depth along the transition. Hence, these factors are not simple functions of central tendencies in water table dynamics but relate to duration of saturated conditions in the near-surface sandy zone.

Mass Loss from LNAPL Pools under Fluctuating Water Table Conditions

Nonaqueous-phase liquids less dense than water (LNAPLs) form pools at the ground-water table, when leaked in sufficient quantities into the subsurface. Subsequent fluctuations of the ground-water table create a smear zone in the vicinity of the water table with the LNAPLs trapped in the pore spaces in the form of blobs. In addition to the pool, these blobs participate in dissolution and biodegradation processes in the saturated zone. A quantitative method is developed to evaluate mass loss of LNAPLs in the saturated zone under fluctuating water table conditions. A mathematical scheme is used to transform the moving domain to a stationary domain and the transformed advective-dispersive-reaction equation is solved numerically. The mass loss is computed in terms of two components: (1) Biodegradation within the domain; and (2) advective flux leaving the domain. The results indicate that the fluctuating water table conditions enhance the mass loss inasmuch as the entrapped blobs contribute to dissolution and b...

The importance of fluid entrapment, saturation hysteresis and residual saturations on the distribution of a lighter-than-water non-aqueous phase liquid in a variably saturated sand medium

A two-dimensional, multiphase flow experiment was conducted in the laboratory and numerically modelled using a finite difference multiphase flow code. Heptane, a lighter-than-water non-aqueous phase liquid (LNAPL), was spilled on a variably saturated sand medium. After the spill was completed, the LNAPL was allowed to distribute itself above the capillary fringe. The water pressure at the base of the experimental box was subsequently raised and lowered on two separate occasions to simulate fluctuating water table conditions. The water and LNAPL pressures were measured using hydrophillic and hydrophobic porous cups connected to pressure transducers and a data acquisition system. The laboratory spill was modelled using a hysteretic and non-hysteretic multiphase flow code. A comparison of the experimental data to the model results illustrates the effects and importance of fluid entrapment and saturation hysteresis.

Key variables controlling the vegetation of a cool‐temperate mire in northern Japan

Abstract. In the cool‐temperate Bibi Mire, Hokkaido, Japan, valley fens and flood‐plain fens have quite different vegetation. The main variables controlling the vegetation were all hydrological: mean water level, water level fluctuation and surface water flow. Chemical factors such as electrical conductivity, dissolved oxygen and related peat decomposition were less important. The pH was about neutral and has little effect. The flood‐plain fen developed under fluctuating water table conditions. The dominant species are Calamagrostis langsdotffii and Carex pseudocuraica. When temporal inundation occurs in the rainy or typhoon seasons, the submergence stimulates bud germination of the stoloniferous C. pseudocuraica, which can rapidly elongate its stolons upward. Some large floating peat mats occurred in the flood‐plain fen zone. On these mats some Alnus japonica saplings establish and patches of alder forest can arise. Here the water level was higher than in the peripheral alder forest zone.The valley fen is dominated by Carex lasiocarpa var. occultans and/or C. limosa. It is formed under stable water table conditions in the inundated parts of the mire ‐where the non‐inundated wet areas are dominated by alder trees. In the area where the surface water is flowing, these two fen sedges grow in deeper water since the high oxygen content is considered to compensate the flooding stress.

LNAPL Distribution in a Cohesionless Soil: A Field Investigation and Cryogenic Sampler

AbstractLighter‐than‐water Non‐Aqueous Phase Liquids (LNAPLs), such as jet fuels or gasolines, are common contaminants of soils and ground water. However, the total volume and distribution of an LNAPL is difficult to accurately determine during a site investigation. LNAPL that is entrapped in the saturated zone due to fluctuating water table conditions is particularly difficult to quantify. Yet, the amount of entrapped product in the saturated zone is theoretically higher, per volume of soil, than the residual product in the unsaturated zone, and small amounts of LNAPL in the saturated zone can contaminate large volumes of ground water.The only method currently available to quantify the amount of LNAPL is direct soil‐core sampling combined with laboratory analysis of the fluid extracted from the soil cores. However, direct sampling of saturated ground water systems with conventional samplers presents a number of problems. In this study, a new sampler was developed that can be used to retrieve undisturbed soil and pore fluid samples from below the water table in cohesionless soils. The sampler uses carbon dioxide to cool the bottom of a saturated soil sample in situ to near freezing. Results of a field study where a prototype sampler was tested demonstrate the usefulness of a cryogenic sampler and show that the amount of LNAPL entrapped below the water table can be a significant part of the total LNAPL in the soil.