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At least 55 records · Page 3Linked to original sources
USGS2016

Structured heterogeneity in a marine terrace chronosequence: Upland mottling

Soil mottles generally are interpreted as a product of reducing conditions during periods of water saturation. The upland soils of the Santa Cruz, CA, marine terrace chronosequence display an evolving sequence of reticulate mottling from the youngest soil (65 ka) without mottles to the oldest soil (225 ka) with well-developed mottles. The mottles consist of an interconnected network of clay and C-enriched regions (gray, 2.5Y 6/1) bordered by leached parent material (white, 2.5Y 8/1) within a diminishing matrix of oxidized parent material (orange, 7.5YR 5/8). The mottles develop in soils that formed from relatively uniform nearshore sediments and occur below the depth of soil bioturbation. To explore how a presumably wetland feature occurs in an unsaturated upland soil, physical and chemical characteristics of mottle separates (orange, gray, and white) were compared through the deep time represented by the soil chronosequence. Mineralogical, isotopic, and surface-area differences among mottle separates indicate that rhizogenic centimeter-scale mass transfer acting across millennia is an integral part of weathering, pedogenesis, and C and nutrient transfer. Elemental analysis, electron microscopy, and Fe-isotope systematics indicate that mottle development is driven by deep roots together with their fungal and microbial symbionts. Taken together, these data suggest that deep soil horizons on old stable landforms can develop reticulate mottling as the long-term imprint of rhizospheric processes. The processes of rhizogenic mottle formation appear to regulate pedogenesis, nutrients, and C sequestration at depth in unsaturated zones.

California↗
USGS2016

Seasonal Variability in Vadose zone biodegradation at a crude oil pipeline rupture site

Understanding seasonal changes in natural attenuation processes is critical for evaluating source-zone longevity and informing management decisions. The seasonal variations of natural attenuation were investigated through measurements of surficial CO2 effluxes, shallow soil CO2 radiocarbon contents, subsurface gas concentrations, soil temperature, and volumetric water contents during a 2-yr period. Surficial CO2 effluxes varied seasonally, with peak values of total soil respiration (TSR) occurring in the late spring and summer. Efflux and radiocarbon data indicated that the fractional contributions of natural soil respiration (NSR) and contaminant soil respiration (CSR) to TSR varied seasonally. The NSR dominated in the spring and summer, and CSR dominated in the fall and winter. Subsurface gas concentrations also varied seasonally, with peak values of CO2 and CH4 occurring in the fall and winter. Vadose zone temperatures and subsurface CO2 concentrations revealed a correlation between contaminant respiration and temperature. A time lag of 5 to 7 mo between peak subsurface CO2 concentrations and peak surface efflux is consistent with travel-time estimates for subsurface gas migration. Periods of frozen soils coincided with depressed surface CO2 effluxes and elevated CO2 concentrations, pointing to the temporary presence of an ice layer that inhibited gas transport. Quantitative reactive transport simulations demonstrated aspects of the conceptual model developed from field measurements. Overall, results indicated that source-zone natural attenuation (SZNA) rates and gas transport processes varied seasonally and that the average annual SZNA rate estimated from periodic surface efflux measurements is 60% lower than rates determined from measurements during the summer.

Vadose Zone Journal↗
USGS2016

Hydrologic impacts of thawing permafrost—A review

Where present, permafrost exerts a primary control on water fluxes, flowpaths, and distribution. Climate warming and related drivers of soil thermal change are expected to modify the distribution of permafrost, leading to changing hydrologic conditions, including alterations in soil moisture, connectivity of inland waters, streamflow seasonality, and the partitioning of water stored above and below ground. The field of permafrost hydrology is undergoing rapid advancement with respect to multiscale observations, subsurface characterization, modeling, and integration with other disciplines. However, gaining predictive capability of the many interrelated consequences of climate change is a persistent challenge due to several factors. Observations of hydrologic change have been causally linked to permafrost thaw, but applications of process-based models needed to support and enhance the transferability of empirical linkages have often been restricted to generalized representations. Limitations stem from inadequate baseline permafrost and unfrozen hydrogeologic characterization, lack of historical data, and simplifications in structure and process representation needed to counter the high computational demands of cryohydrogeologic simulations. Further, due in part to the large degree of subsurface heterogeneity of permafrost landscapes and the nonuniformity in thaw patterns and rates, associations between various modes of permafrost thaw and hydrologic change are not readily scalable; even trajectories of change can differ. This review highlights promising advances in characterization and modeling of permafrost regions and presents ongoing research challenges toward projecting hydrologic and ecologic consequences of permafrost thaw at time and spatial scales that are useful to managers and researchers.

Vadose Zone Journal↗
USGS2018

Imidacloprid sorption and transport in cropland, grass buffer and riparian buffer soils

An understanding of neonicotinoid sorption and transport in soil is critical for determining and mitigating environmental risk associated with the most widely used class of insecticides. The objective of this study was to evaluate mobility and transport of the neonicotinoid imidacloprid (ICD) in soils collected from cropland, grass vegetative buffer strip (VBS), and riparian VBS soils. Soils were collected at six randomly chosen sites within grids that encompassed all three land uses. Single-point equilibrium batch sorption experiments were conducted using radio-labeled ( 14 C) ICD to determine solid–solution partition coefficients ( K d ). Column experiments were conducted using soils collected from the three vegetation treatments at one site by packing soil into glass columns. Water flow was characterized by applying Br − as a nonreactive tracer. A single pulse of 14 C-ICD was then applied, and ICD leaching was monitored for up to 45 d. Bromide and ICD breakthrough curves for each column were simulated using CXTFIT and HYDRUS-1D models. Sorption results indicated that ICD sorbs more strongly to riparian VBS ( K d = 22.6 L kg −1 ) than crop ( K d = 11.3 L kg −1 ) soils. Soil organic C was the strongest predictor of ICD sorption ( p < 0.0001). The column transport study found mean peak concentrations of ICD at 5.83, 10.84, and 23.8 pore volumes for crop, grass VBS, and riparian VBS soils, respectively. HYDRUS-1D results indicated that the two-site, one-rate linear reversible model best described results of the breakthrough curves, indicating the complexity of ICD sorption and demonstrating its mobility in soil. Greater sorption and longer retention by the grass and riparian VBS soils than the cropland soil suggests that VBS may be a viable means to mitigate ICD loss from agroecosystems, thereby preventing ICD transport into surface water, groundwater, or drinking water resources.

Vadose Zone Journal↗
USGS2018

Unsaturated zone CO2, CH4, and δ13C-CO2 at an arid region low-level radioactive waste disposal site

Elevated tritium, radiocarbon, Hg, and volatile organic compounds associated with low-level radioactive waste (LLRW) at the USGS Amargosa Desert Research Site (ADRS) have stimulated research on factors and processes that affect contaminant gas distribution and transport. Consequently, we examined the sources, mixing, and biogeochemistry of CO 2 and CH 4 , two additional important species in the unsaturated zone at ADRS. In spring 2015 and 2016, shallow unsaturated zone gas samples were collected from the 1.5-m depth both inside and outside the LLRW disposal area. Samples also were collected from two 110-m-deep multilevel gas-sampling boreholes and a distant background site. These samples were analyzed for CO 2 mole fraction ( x CO 2 ) and C isotopic composition (δ 13 C-CO 2 ) and CH 4 mole fraction ( x CH 4 ). Graphical analysis of the results indicates mixing of CO 2 characteristic of the root zone (δ 13 C −18 to −19‰), deep soil gas of the capillary fringe (−13 to −15‰), and CO 2 produced by microbial respiration of organic matter disposed in the LLRW trenches (−22 to −25‰). Distribution of CH 4 overall reflects atmospheric sources and production in anaerobic microzones in the LLRW area and methanotrophy in the undisturbed shallow subsurface outside the LLRW area. Although x CH 4 reflecting lateral transport from the LLRW area is decreasing with time in the deep profiles, deep unsaturated zone x CO 2 has changed little in recent decades. The results imply that CH 4 and δ 13 C-CO 2 may serve as good tracers of anthropogenic effects in the unsaturated zone even when CO 2 primarily reflects natural processes.

Nevada↗
USGS2018

Specific conductance as a tracer of preferential flow in a subsurface-drained field

Specific conductance (SC), soil volumetric water content (VWC), and discharge were monitored on a subsurface agricultural drain for a 2-yr period (2007–2008) to differentiate preferential flow paths from matrix flow paths. A major observation from the 2-yr period was the fast SC decrease after relatively small rainfall events, often <5 mm. A total of 25 paired rainfall–SC events were classified, with an average preferential flow onset time (from the event start) after 1.7 h and maximum preferential flow after 2.4 h. A specific conductance end-member mixing analysis (SC-EMMA) was used to determine the volume of water that infiltrated through preferential flow pathways. The SC-EMMA was used for 20 of the 25 paired rainfall–SC events; of the 20 classified events, the maximum preferential flow ranged from 11 to 75% of the total subsurface drain flow, with a mean maximum preferential flow of 31%. Overall, SC-EMMA illustrated that a significant portion of the subsurface drain discharge can be attributed to preferential flow, mainly through macropores or other largely open preferential flow pathways. The other primary mechanism, antecedent moisture conditions shifts, could only be shown for four of the 25 classified events. Specific conductance as a tracer of preferential flow was shown to be an effective tool for distinguishing preferential flow to subsurface drains. Even during relatively dry periods, the SC had a substantial decrease shortly after a rainfall event contrary to the conventional idea that macropore flow starts only after all the smaller pores are saturated and surface ponding begins to occur.

Iowa↗
USGS2018

Ecohydrologic changes caused by hydrologic disconnection of ephemeral stream channels in Mojave National Preserve, California

Emplacement of highways and railroads has altered natural hydrologic systems by influencing surface-water flow paths and biotic communities in Mojave National Preserve. Infiltration experiments were conducted along active and abandoned channels to evaluate changes in hydrology and related effects on plant water availability and use. Simulated rainfall infiltration experiments with vegetation monitoring were conducted along an active channel upslope and a comparable abandoned channel down slope of the transportation corridor. We also conducted 90 single-ring, ponded infiltration experiments in adjacent channels to evaluate field-saturated hydraulic conductivity and particle size distributions. The abandoned channels are still morphologically evident, but are disconnected from runoff sources at higher elevations. Infiltration test results show that water infiltrates twice as fast in the active channels. Excavation showed weak soil development with fewer plant roots beneath the abandoned channel. SEM analysis on surface samples showed the presence of cyanobacteria only in abandoned channels. Plants up to three meters away from both channels showed physiological responses to channel water applied in a simulated pulse of rain. The response was short-lived and less pronounced for plants adjacent to the abandoned channel, whereas those adjacent to the active channel showed responses up to two months after the pulse. These responses may explain observed lower plant densities and fewer deep-rooted species along abandoned channels compared to active channels. We infer that the deeper-rooting plants are more abundant where they are able to take advantage of the increased soil-water storage resulting from greater infiltration and flow frequency in active stream channels.

California↗
USGS2018

A direct-push freezing core barrel for sampling unconsolidated subsurface sediments and adjacent pore fluids

Contaminants passing through the unsaturated zone can undergo changes in narrow reaction zones upon reaching saturated sediments. Understanding these reactions requires sampling of sediment together with adjacent water and microbes in a manner that preserves in situ redox conditions. Use of a basket-type core catcher for saturated, noncohesive sediments results in redistribution or loss of fluids during sample retrieval. Previously developed sample-freezing drive shoes for hollow-stem auger drilling rigs lessened fluid redistribution and retained all material that entered the core barrel in noncohesive sediment cores by freezing the base of the core with liquid CO 2 . This technology has not previously been compatible with direct-push rigs that are commonly used for contaminated site assessments. Here, we describe a freezing core barrel designed for direct-push rigs that is compatible with commercially available tool strings. The device can be used interchangeably with unsaturated-zone direct-push tool strings, enabling core collection for studies of contaminant transport and transformation spanning unsaturated to saturated profiles. In all 10 attempts during testing near Bemidji, MN, the device froze a 10- to 15-cm (4–6-in) plug that retained fluids and sediments in a 1.2-m (4-ft)-long, 5.0-cm (2.0-in)-diameter polyvinyl chloride (PVC) sleeve. Cores were collected from variably saturated sediments spanning the capillary fringe through the upper 2 m of the saturated zone in sandy glacial outwash sediments. The median recovery was 81% of the drive length, similar to a sample-freezing drive shoe developed for a wire-line piston core sampler operated with a hollow-stem auger drill rig.

Vadose Zone Journal↗
USGS2018

Filtering of cyclic period infiltration in a layered vadose zone: 1. Approximation of damping and time lags

Core Ideas We describe an approximation for filtering of periodic infiltration in layered soil. Transitions in soil‐water properties between soil layers affect the filtering. Errors are smaller in soils where changes in soil‐water properties are small. Infiltration and downward percolation of water in the vadose zone are important processes that can define the availability of water resources. We present an approach that provides insight into how periodic infiltration forcings at the land surface filter in a layered vadose zone in terms of changes in the timing and magnitude of hydrologic responses. To represent geologically realistic systems, we used vertical sequences of one‐dimensional periodic solutions, where each solution represents a single soil in a layered profile. The overall approach is based on a linearized Richards equation and assumes that the effects on flow of continuous pressure head changes at soil interfaces are negligible. We evaluated the limit of these approximations by comparison with results from the numerical model HYDRUS‐1D, which uses the full Richards equation. We compared (i) the depth at which flux variations became steady, and (ii) the travel time of wetting fronts to reach a depth of 3 m. The solution was reasonably accurate (error less than a factor of 2) for infiltration cycles with periods from 30 to 365 d and for fluxes common in arid and semiarid environments (0–2 mm d −1 ). Lag times between a surface forcing and response at any depth were accurate (error less than a factor of 1.1). The approximation generally provided consistent estimates of the damping and time lag, such that it overestimated the depths where fluxes were steady and underestimated the time for a forcing to reach a specific depth.

Vadose Zone Journal↗
USGS2018

Filtering of periodic infiltration in a layered vadose zone: 2. Applications and a freeware screening tool

We have developed a screening tool to visualize and conceptualize the filtering properties of a layered vadose zone. Climate projections indicate that rainfall timing and magnitude may change and impact groundwater resources. This increases the importance of understanding how the vadose zone filters infiltration variability and ultimately affects recharge and groundwater resources. An approximate solution for the filtering of surface forcings through soil layers was developed previously, and the soil and conditions where its approximations are appropriate was evaluated. Here we present a screening tool based on the solution for estimating how periodic infiltration forcings filter in a layered vadose zone for different soil properties and surface flux conditions. The solutions identify time-varying elements of surface forcings that persist to the depth of the water table, leading to transient recharge. We investigated the filtering properties of the vadose zone in Central Valley, California, and identified areas where surface forcings are essentially damped and recharge can be approximated as steady. We also determined the travel time for infiltration pulses to reach the depth of the water table.

Vadose Zone Journal↗
USGS2018

Episodic master recession evaluation of groundwater and streamflow hydrographs for water-resource estimation

Hydrograph analysis tools using a master recession curve (MRC) can produce many types of hydrologically important watershed-response quantifications, including aquifer recharge and stormflow characterization. An MRC is the relation between the value of a measured response R and its rate of change with time, d R /d t , occurring on the falling limb when there is no infiltration or other water input. We have developed MRC and episodic hydrograph-evaluation methods for multiple purposes, utilizing both water table and streamflow data. The determination of a parameterized MRC through a structured procedure provides a basis for quantification of hydrologic variables and characteristics that can be validly compared among different events, sites, and periods of time. Application of the MRC to needed hydrologic quantifications is done with our revised episodic master recession (EMR) method. Expert-guided iterative procedures are used to quantify parameters needed in applying the MRC and EMR methods to a given site. Hydrologic judgments such as the significance threshold for response magnitude, and the time window within which the precipitation is assumed to be the cause of an observed response, inherently involve some elements of subjectivity. Our structured iterative approach, however, affords much flexibility in formulating expert judgments and serves to confine them to statements and procedures that can be quantified and documented. Parallel application to streamflow and water table hydrographs can produce new hydrologic insights and understanding, not least in the role of unsaturated zone processes in controlling exchanges among components of the water cycle.

Vadose Zone Journal↗
USGS2018

Long-term soil-water tension measurements in semi-arid environments: A method for automated tensiometer refilling

Tensiometer-equipped data acquisition systems measure and record positive and negative soil-water pressures. These data contribute to studies in hillslope hydrology, including analyses of rainfall runoff, near-surface hydrologic response, and slope stability. However, the unique ability of a tensiometer to rapidly and accurately measure pre- and post-saturation subsurface pressures requires maintenance techniques that have precluded their application to unattended sensor networks in semiarid regions. Under suction, the de-aired water in the tensiometer is drawn from a porous cup. Under positive pressure, dissolved gases from pore water infiltrates the cup. Over time, both contribute to unreliable readings and/or poor signal response through cavitation. To address this problem, we used commercially available equipment to develop a simple system of solenoid valves and a water reservoir that enable automated in situ tensiometer refilling. We tested the system at two post-wildfire hydrologic monitoring sites in the Angeles National Forest, southern California. We present example results from 3 mo of monitoring and show how the tensiometers can be refilled by a remote trigger. By remotely refilling the tensiometer, we were able to continuously monitor quasi-saturated soil pore-water pressures without making repeated and costly maintenance visits.

Vadose Zone Journal↗
USGS2018

Field and laboratory hydraulic characterization of landslide-prone soils in the Oregon Coast Range and implications for hydrologic simulation

Unsaturated zone flow processes are an important focus of landslide hazard estimation. Differences in soil hydraulic behavior between wetting and drying conditions (i.e., hydraulic hysteresis) may be important in landslide triggering. Hydraulic hysteresis can complicate soil hydraulic parameter estimates and impact prediction capability. This investigation focused on hydraulic property estimation for soil in a landslide‐prone area where the relative importance of hysteresis is unclear. Laboratory measurements of soil‐water retention from field soils in the Oregon Coast Range during wetting and drying show that pronounced hydraulic hysteresis is present. In contrast, a 4‐yr field data record of pore‐water pressure and soil‐water content from multiple soil pits at the same landslide‐prone area shows relatively minor hydraulic hysteresis compared with the laboratory estimates. Simulated subsurface hydrologic response parameterized using estimates from field data more closely matched hydrologic observations relative to model parameterization based on laboratory analysis of repacked soil samples. Our results suggest that (i) unsaturated hydraulic parameter estimates based on in situ field data, as opposed to laboratory measurements alone, may lead to more accurate simulation of the hydrologic response to rainfall, (ii) in situ data of soil‐water retention may need to include values at both high suctions and near saturation to improve estimates of soil hydraulic parameters for slope failure applications, and (iii) laboratory measurements of soil‐water retention made under dynamic conditions may overestimate hydraulic hysteresis.

Oregon↗
USGS2019

Spatial fingerprinting of biogenic and anthropogenic volatile organic compounds in an arid unsaturated zone

Subsurface volatile organic compounds (VOCs) can pose risks to human and environmental health and mediate biological processes. VOCs have both anthropogenic and biogenic origins, but the relative importance of these sources has not been explored in subsurface environments. This study synthesizes 17 years of VOC data from the Amargosa Desert Research Site (ADRS) with the goal of improving understanding of spatial and temporal variations that distinguish sources of VOCs from a landfill and surrounding ambient sources including biogenic VOCs (bVOCs). Gas samples were collected from 1999 to 2016 from an array of shallow sample points (0.5 m and 1.5 m depth) and from vertical profiles at three deep boreholes, two (109 m deep) near the border of a waste facility (33 and 100 m distant), and one (29 m deep) in a remote area 3 km to the south. Samples were analyzed for target VOCs and a subset was analyzed for non-target VOCs to enumerate a greater variety of potential bVOCs. Principal components analysis of the target and non-target VOCs provided an assessment of spatial variability of VOCs originating from the landfill site and from ambient sources. Ambient VOCs occurred at all sample sites over a range of depths and most were consistent with biogenic origins, indicating, for the first time, presence of bVOCs in the deep unsaturated zone. Because some VOCs have both anthropogenic and biogenic sources, discrimination of sources can be important for estimating the extent and migration of anthropogenic plumes in arid unsaturated zones.

Nevada↗
USGS2018

Climate variability and vadose zone controls on damping of transient recharge

Increasing demand on groundwater resources motivates understanding of the controls on recharge dynamics so model predictions under current and future climate may improve. Here we address questions about the nonlinear behavior of flux variability in the vadose zone that may explain previously reported teleconnections between global-scale climate variability and fluctuations in groundwater levels. We use hundreds of HYDRUS-1D simulations in a sensitivity analysis approach to evaluate the damping depth of transient recharge over a range of periodic boundary conditions and vadose zone geometries and hydraulic parameters that are representative of aquifer systems of the conterminous United States (U.S). Although the models were parameterized based on U.S. aquifers, findings from this study are applicable elsewhere that have mean recharge rates between 3.65 and 730 mm yr –1 . We find that mean infiltration flux, period of time varying infiltration, and hydraulic conductivity are statistically significant predictors of damping depth. The resulting framework explains why some periodic infiltration fluxes associated with climate variability dampen with depth in the vadose zone, resulting in steady-state recharge, while other periodic surface fluxes do not dampen with depth, resulting in transient recharge. We find that transient recharge in response to the climate variability patterns could be detected at the depths of water levels in most U.S. aquifers. Our findings indicate that the damping behavior of transient infiltration fluxes is linear across soil layers for a range of texture combinations. The implications are that relatively simple, homogeneous models of the vadose zone may provide reasonable estimates of the damping depth of climate-varying transient recharge in some complex, layered vadose zone profiles.

Journal of Hydrology↗
USGS2012

Roles of saltcedar (Tamarix spp.) and capillary rise in salinizing a non-flooding terrace on a flow-regulated desert river

Tamarix spp. (saltcedar) secretes salts and has been considered to be a major factor contributing to the salinization of river terraces in western US riparian zones. However, salinization can also occur from the capillary rise of salts from the aquifer into the vadose zone. We investigated the roles of saltcedar and physical factors in salinizing the soil profile of a non-flooding terrace at sites on the Cibola National Wildlife Refuge on the Lower Colorado River, USA. We placed salt traps under and between saltcedar shrubs and estimated the annual deposition rate of salts from saltcedar. These were then compared to the quantities and distribution on of salts in the soil profile. Dense stands of saltcedar deposited 0.159 kg m −2 yr −1 of salts to the soil surface. If this rate was constant since seasonal flooding ceased in 1938 and all of the salts were retained in the soil profile, they could account for 11.4 kg m −2 of salt, about 30% of total salts in the profile today. Eliminating saltcedar would not necessarily reduce salts, because vegetation reduces the upward migration of salts in bulk flow from the aquifer. The densest saltcedar stand had the lowest salt levels in the vadose zone in this study.

California↗
USGS2021

Unsaturated flow processes and the onset of seasonal deformation in slow-moving landslides

Predicting rainfall-induced landslide motion is challenging because shallow groundwater flow is extremely sensitive to the preexisting moisture content in the ground. Here, we use groundwater hydrology theory and numerical modeling combined with five years of field monitoring to illustrate how unsaturated groundwater flow processes modulate the seasonal pore water pressure rise and therefore the onset of motion for slow-moving landslides. The onset of landslide motion at Oak Ridge earthflow in California’s Diablo Range occurs after an abrupt water table rise to near the landslide surface 52–129 days after seasonal rainfall commences. Model results and theory suggest that this abrupt rise occurs from the advection of a nearly saturated wetting front, which marks the leading edge of the integrated downward flux of seasonal rainfall, to the water table. Prior to this abrupt rise, we observe little measured pore water pressure response within the landslide due to rainfall. However, once the wetting front reaches the water table, we observe nearly instantaneous pore water pressure transmission within the landslide body that is accompanied by landslide acceleration. We cast the timescale to reach a critical pore water pressure threshold using a simple mass balance model that considers variable moisture storage with depth and explains the onset of seasonal landslide motion with a rainfall intensity-duration threshold. Our model shows that the seasonal response time of slow-moving landslides is controlled by the dry season vadose zone depth rather than the total landslide thickness.

California↗
USGS1979

Fresh-water cementation of a 1,000-year-old oolite

Calcite cementation of aragonite ooid sand is producing oolite on Joulters Cays, Bahamas. During the last 1,000 years, calcite cement has formed at an average rate of between 27 and 55 cm 3 /m 3 /yr and is derived from dissolution of ooid aragonite in fresh water. The dissolution-reprecipitation of carbonate minerals in the aquifer results in ground waters of unusually high Sr content. Sea water and mixtures of fresh and sea water appear to inhibit cementation. A pronounced cement fabric change occurs across the water table and has produced an obvious petrographic record of fresh-water diagenesis. Above the water table, cement is typically near grain contact positions, where water is held by capillarity; below the water table, cement is more randomly distributed around grains. At the water table a transition zone, 1 meter thick, marks the boundary between cement textures. No porosity reduction is associated with cementation; calcite cement precipitation is apparently compensated by an equal or greater amount of aragonite dissolution in the interval undergoing cementation. Permeability is more variable above the water table than below it, reflecting early channelling of flow patterns in the vadose zone. Effective permeability below the water table is one to two orders of magnitude higher than above the water table because of entrained gas in the vadose zone. This permeability difference promotes preservation of unstable minerals above the water table and continued diagenetic alteration below the water table.

Joulters Cays↗