Alicia M. Wilson

The effects of pulsed pumping on land subsidence in the Santa Clara Valley, California

Abstract Land subsidence caused by pulsed and constant pumping schemes was calculated, and the results were compared to determine the possible advantages of pulsed pumping in reducing land subsidence. Pulsed pumping refers to pumping strategies involving alternation between periods of pumping and recovery in a well. Subsidence was calculated using a numerical model based on the Santa Clara Valley, CA. The model accounts for delay in the release of water from storage in compressible interbeds as well as flow within aquifers. For these tests, the ratio of pumping period length to recovery period length was either 1 1 or 1 3 . Pulsing interval length (length of one pumping period plus one recovery period) ranged from 4 to 90 days. Pumping rates were adjusted so that pulsed pumping and constant pumping removed the same volume of water over the period under consideration. Pulsed pumping has the potential to ‘concentrate’ subsidence around the pumping well. For all simulations, pulsed pumping caused greater subsidence near the test well than constant pumping. Under some conditions a transition point was reached, beyond which pulsed pumping caused less subsidence than steady pumping. The distance from the test well to the transition point decreases for (a) decreasing aquifer transmissivity, (b) decreasing ratio of pumping period to recovery period, and (c) increasing pulsing interval length. The concentration of subsidence was reduced considerably when delay in release of water from storage in compressible aquitards was neglected, indicating the importance of including delay effects in subsidence studies. All pulsed pumping strategies caused a greater total volume of subsidence than steady pumping.

The influence of tidal forcing on groundwater flow and nutrient exchange in a salt marsh-dominated estuary

Data from salt marshes in the U.S. Southeast show that long-term variations in mean water level (MWL) correlate strongly with salt marsh productivity and porewater salinity. Here we used numerical models of tidally-driven groundwater flow to assess the effect of variations in tidal amplitude and MWL on porewater exchange between salt marshes and tidal creeks. We modeled homogeneous and layered stratigraphy and compared flat and sloped topography for the marsh surface. Results are consistent with field observations and showed that increases in tidal amplitude increased groundwater flushing, particularly when increasing the tidal amplitude caused the marsh platform to be inundated at high tide. Increases in MWL caused groundwater flushing to increase if that rise caused greater areas of the marsh to be inundated at high tide. Once the marsh was fully inundated at high tide, further increases in MWL caused groundwater flushing to decrease. Results suggest that small increases in MWL associated with sea level rise could increase nutrient export significantly in marshes with elevations that are equilibrated near mean high water, but rising sea level could decrease the export of nutrients to, and thus fertility in, estuaries adjacent to marshes that are equilibrated lower in the tidal frame. Likewise, macrotidal estuaries are predicted to be subject to much larger groundwater and nutrient exchange than similar microtidal estuaries. We speculate that the early stages of rising relative sea level may significantly impact water quality in estuaries that are not river-dominated by raising the discharge of nutrients from coastal wetlands.

Groundwater controls ecological zonation of salt marsh macrophytes

Ecological zonation of salt marsh macrophytes is strongly influenced by hydrologic factors, but these factors are poorly understood. We examined groundwater flow patterns through surficial sediments in two saltmarshes in the southeastern United States to quantify hydrologic differences between distinct ecological zones. Both sites included tall- or medium-form Spartina alterniflora near the creek bank; short-form Spartina alterniflora in the mid-marsh; salt flats and Salicornia virginica in the high marsh; and Juncus roemarianus in brackish-to-fresh areas adjacent to uplands. Both sites had relatively small, sandy uplands and similar stratigraphy consisting of marsh muds overlying a deeper sand layer. We found significant hydrologic differences between the four ecological zones. In the zones colonized by S. alterniflora, the vertical flow direction oscillated with semi-diurnal tides. Net flow (14-day average) through the tall S. alterniflora zones was downward, whereas the short S. alterniflora zones included significant periods of net upward groundwater flow. An examination of tidal efficiency at these sites suggested that the net flow patterns rather than tidal damping controlled the width of the tall S. alterniflora zone. In contrast to the S. alterniflora zones, hypersaline zones populated by S. virginica were characterized by sustained periods (days) of continuous upward flow of saline water during neap tides. The fresher zone populated by J. roemarianus showed physical flow patterns that were similar to the hypersaline zones, but the upwelling porewaters were fresh rather than saline. These flow patterns were influenced by the hydrogeologic framework of the marshes, particularly differences in hydraulic head between the upland water table and the tidal creeks. We observed increases in hydraulic head of approximately 40 cm from the creek to the upland in the sand layers below both marshes, which is consistent with previous observations that sandy aquifers below fine-grained marsh soils act as conduits for flow from uplands to tidal creeks. This hydrologic framework supports relatively good drainage near the creek, increased waterlogging in the mid-marsh, and the development of hypersalinity adjacent to the freshwater upland. These hydrologic differences in turn support distinct ecological zones.

What time scales are important for monitoring tidally influenced submarine groundwater discharge? Insights from a salt marsh

Submarine groundwater discharge (SGD) varies significantly across time scales ranging from hours to years, but studies that allow quantitative comparisons between different time scales are few. Most of these studies have focused on beach settings, where the combined variations in fresh and saline SGD can be difficult to interpret. We calculated variations in saline SGD based on a 1 year record of hydraulic head in a salt marsh, where we could isolate variations in saline, tidally driven SGD. Observed SGD varied by an order of magnitude over the course of the year. Groundwater discharge was proportional to tidal amplitude and varied by at least a factor of 2 between spring and neap tides. Monthly average SGD was inversely proportional to average sea level; it increased by nearly a factor of 2 as sea level declined by ∼50 cm from late summer to late winter. This variation was far larger than that predicted by analytic models, owing to the flat topography and inundation of the marsh platform. The effect of short-term (days) variations in sea level associated with wind events and storms was small in comparison. SGD is probably proportional to tidal amplitude in nearly all coastal settings, including beaches. Seasonal variations in sea level may not affect the volume of SGD as significantly in coastal settings where the slope of the intertidal zone is relatively constant, but such variations have the potential to strongly affect the composition of SGD.

Comment on “Subsurface flow and vegetation patterns in tidal environments” by Nadia Ursino, Sonia Silvestri, and Marco Marani

] Ursino et al. [2004] suggested that aeration timein salt-marsh sediments could affect plant zonation andpresented numerical models of tidally driven groundwaterflow to estimate aeration time in a theoretical creek bank.They found that under conditions of high ET loss theduration of aeration can increase with distance from thecreek bank. If so, this might lead to greater marsh grassproductivity with distance from the creek rather than less asis commonly observed [Dame and Kenny, 1986]. Aerationtime is, of course, only one of many factors that could affectmarsh productivity, including salinity, sulfide concentration,and nutrient transport, but we agree that aeration likelyplays an important role in tidal marsh ecology. Therefore thepotentially contradictory finding that aeration time couldincrease with distance from the creek warrants closerexamination. Unfortunately, the model of Ursino et al. isnot clearly described and appears to be based on assump-tions that are not realistic.[

Cl/Br compositions as indicators of the origin of brines: Hydrogeologic simulations of the Alberta Basin, Canada

The origin and residence time of brines in the Alberta Basin have been debated for more than 40 years, with conflicting conclusions reported by geochemical and hydrogeologic studies. Here, numerical models were used to determine hydrogeologically feasible scenarios for the origin of brines in the Alberta Basin, using salinity and Cl/Br ratios as geochemical constraints. The models simulated variable-density fluid flow, heat transport, solute transport, and sediment compaction and decompaction in the Alberta Basin over the last 100 m.y. Simulation results suggest that pore fluids in this basin represent a mixture of four geochemical end members: seawater, freshwater, brines formed by evaporation of seawater, and, contrary to prior interpretations of Cl/Br ratios, brines derived from halite dissolution. Sensitivity studies revealed that similar distributions of salinity and Cl/Br ratios could be obtained without dissolution of halite if extremely low permeabilities were used in the model, but this scenario conflicts with field-based permeability data and prior simulations of petroleum migration in the basin. The residence time of brines in the Alberta Basin has thus likely been overestimated. The presence of evaporites introduces significant uncertainty in the use of Cl/Br ratios for interpreting the origin of brines in sedimentary basins, but salinity and Cl/Br ratios provide valuable new constraints for regional-scale models.

Calcium mass transport and sandstone diagenesis during compaction-driven flow: Stevens Sandstone, San Joaquin basin, California

Sediment compaction provides a limited source of fluids for diagenesis and drives fluid flow at average rates of only ∼1 mm/yr. Nevertheless, many geochemical and petrographic studies of diagenesis provide evidence of significant mass transport in systems where fluid flow appears to be compaction driven. This apparent discrepancy could arise because diagenetic studies generally are concerned with relatively permeable petroleum reservoirs. Focused fluid flow through permeable zones could increase local flow rates and allow mixing of fluids from different sources. This study uses the Stevens Sandstone of the San Joaquin basin to explore the potential diagenetic effects of fluid focusing during compaction-driven flow over a 5 m.y. period. Reactive-transport simulations incorporate a new kinetic expression for plagioclase dissolution and suggest that rate-limited plagioclase dissolution drove calcium enrichment of pore fluids and caused precipitation of calcite, kaolinite, and albite. In spite of the importance of this rate-limited reaction, simulations show that influx of fluids from compacting shale could limit the distribution of calcite and kaolinite in adjacent sandstones. Although calcium mass transport is predicted on a scale of kilometers, upward flow of calcium-rich fluids from deep beds does not significantly increase calcite volume relative to closed-system predictions. Increased transverse dispersivity increases mixing, which further limits precipitation of calcite and kaolinite. Results are consistent with field observations of fluid chemistry, although simulations account for

Science, society, and the coastal groundwater squeeze

Coastal zones encompass the complex interface between land and sea. Understanding how water and solutes move within and across this interface is essential for managing resources for society. The increasingly dense human occupation of coastal zones disrupts natural groundwater flow patterns and degrades freshwater resources by both overuse and pollution. This pressure results in a “coastal groundwater squeeze,” where the thin veneers of potable freshwater are threatened by contaminant sources at the land surface and saline groundwater at depth. Scientific advances in the field of coastal hydrogeology have enabled responsible management of water resources and protection of important ecosystems. To address the problems of the future, we must continue to make scientific advances, and groundwater hydrology needs to be firmly embedded in integrated coastal zone management. This will require interdisciplinary scientific collaboration, open communication between scientists and the public, and strong partnerships with policymakers.

Intense nitrogen cycling in permeable intertidal sediment revealed by a nitrous oxide hot spot

Approximately 40% of the total global rate of nitrogen fixation is the result of human activities, and most of this anthropogenic nitrogen is used to fertilize agricultural fields. Approximately 23% of the applied agricultural nitrogen is delivered to the coastal zone, often reducing water quality and driving eutrophication. Nitrogen cycling in coastal sediments can mitigate eutrophication by removing bioavailable nitrogen. However, some of these processes generate nitrous oxide, a potent greenhouse gas, as a by-product. Here we report the discovery of a nitrous oxide production hot spot in shallow barrier island sands. Nitrous oxide concentrations, production and consumption rates, vertical diffusion fluxes, and flux to the atmosphere were measured across triplicate depth profiles. Using a mass balance approach, rates of net nitrous oxide production were estimated to be 40 µmol m−2 d−1. This production was driven by a hot spot of nitrate consumption that removed bioavailable nitrogen from the coastal environment at a rate of 10 mmol m−2 d−1, a rate that is comparable with the highest rates of denitrification reported for coastal sediments.

Nitrogen fate in a subtropical mangrove swamp: Potential association with seawater-groundwater exchange

Coastal mangrove swamps play an important role in nutrient cycling at the land-ocean boundary. However, little is known about the role of periodic seawater-groundwater exchange in the nitrogen cycling processes. Seawater-groundwater exchange rates and inorganic nitrogen concentrations were investigated along a shore-perpendicular intertidal transect in Daya Bay, China. The intertidal transect comprises three hydrologic subzones (tidal creek, mangrove and bare mudflat zones), each with different physicochemical characteristics. Salinity and hydraulic head measurements taken along the transect were used to estimate the exchange rates between seawater and groundwater over a spring-neap tidal cycle. Results showed that the maximum seawater-groundwater exchange occurred within the tidal creek zone, which facilitated high-oxygen seawater infiltration and subsequent nitrification. In contrast, the lowest exchange rate found in the mangrove zone caused over-loading of organic matter and longer groundwater residence times. This created an anoxic environment conducive to nitrogen loss through the anammox and denitrification processes. Potential oxidation rates of ammonia and nitrite were measured by the rapid and high-throughput method and rates of denitrification and anammox were measured by the modified membrane inlet mass spectrometry (MIMS) with isotope pairing, respectively. In the whole transect, denitrification accounted for 90% of the total nitrogen loss, and anammox accounted for the remaining 10%. The average nitrogen removal rate was about 2.07g per day per cubic meter of mangrove sediments.