Geoelectrical signals of geologic and hydrologic processes in a fringing reef lagoon setting

Coastal groundwater may discharge into nearshore and offshore waters forced by terrestrial fluxes, controlled by local geology, and modulated by the hydrodynamics of littoral water. We investigated the electrical signature of these features with a dense, multiscale network of electrical resistivity tomography (ERT) surveys in the Muri Lagoon of Rarotonga, Cook Islands. The ERT surveys spanned from onshore to 400 m into the lagoon and used standard electrodes on land and across the foreshore, submerged electrodes in the shallow subtidal zone, and floating electrodes towed throughout the reef lagoon by a boat. ERT surveys on land mapped a typical freshwater lens underlain by a saltwater wedge, but with possible deviations from the classical model due to an adjacent tidal creek. Further inland, ERT surveys imaged a layer of lava flow deposits that is potentially a confining hydrogeologic unit; this unit was used to constrain the expected electrical resistivity of these deposits below the lagoon. ERT surveys across the intertidal zone and into the lagoon indicated fresh groundwater and porewater salinity patterns consistent with previous small-scale studies including the seaward extension of fresh groundwater pathways to the lagoon. Electrical resistivity (ER) variations in the lagoon subsurface highlighted heterogeneities in the lagoon structure that may focus submarine groundwater discharge (SGD) through previously unknown buried lava flow deposits in the lagoon. A transition to higher ER values near the reef crest is consistent with the ER signature of porosity reduction due to ongoing differential cementation of reef deposits across the lagoon. The imaged coastal hydrostratigraphic heterogeneity may thus control terrestrial and marine porewater mixing, support SGD, and provide the pathways for groundwater and the materials it transports into the lagoon. This hydrogeophysical investigation highlighted the spatial heterogeneity of submarine coastal geology and its hydrogeologic control in a reef lagoon setting, but is likely to occur in many similar coastal settings. Ignoring geologic complexity can result in mischaracterization of SGD and other coastal groundwater processes at many spatial scales

Nutrient and greenhouse gas dynamics through a range of wastewater-loaded carbonate sand treatments

Most countries in the South Pacific rely on traditional soak pits which transfers wastewater down through the unsaturated carbonate sand sediment profile with little nutrient attenuation. With surface nutrients having an almost direct path to groundwater, this may have consequences for the sustainability of subsurface water reserves. This study examined the long-term removal and production of wastewater nutrients (including greenhouse gases) in tropical sediments and materials under different saturation regimes. The study showed that unsaturated carbonate sands alone are not an effective means of reducing nutrient loads to groundwater. There was no significant difference in removal rates of total dissolved nitrogen (TN) and phosphorus (TP) in unsaturated carbonate sand columns and columns amended with coconut husk and basalt sediment. Doubling of the effluent flow rate resulted in no significant decrease in the TN removal rate, but did halve the TP removal rate. The addition of a biochar filter to the unsaturated carbonate sand columns increased TN removal from ∼8% to ∼42%. The TN removal rate in the wastewater-saturated treatments (662.1 ± 49.4 mg m−3 day−1) was approximately six times higher than the unsaturated treatments (110.2 ± 27.2 mg m−3 day−1). TN removal in the fully saturated columns increased from 63% to 95% after the addition of an external carbon source (glucose). Higher nitrous oxide (N2O) and methane (CH4) concentrations in partially saturated columns and at the surface of the fully saturated columns were likely due to partial coupled nitrifier denitrification. The addition of basalt sediments to carbonate sands increased the total phosphorous removal from 72% to 95%. Sampling of groundwater below an in situ wastewater treatment system showed an almost complete removal of TN and TP within two meters of the aquifer surface, even with limited bioavailable carbon to facilitate denitrification. Modelling of NO3− and N2 concentrations for an idealized treatment system indicated reduced NO3− concentrations may be due to a combination of dilution with high volumes of groundwater (∼45% of removal at the farthest sampling point from the system) and denitrification

DataSheet_1_An integrated approach for physical, economic, and demographic evaluation of coastal flood hazard adaptation in Santa Monica Bay, California.pdf

The increased risk of coastal flooding associated with climate-change driven sea level rise threatens to displace communities and cause substantial damage to infrastructure. Site-specific adaptation planning is necessary to mitigate the negative impacts of flooding on coastal residents and the built environment. Cost-benefit analyses used to evaluate coastal adaption strategies have traditionally focused on economic considerations, often overlooking potential demographic impacts that can directly influence vulnerability in coastal communities. Here, we present a transferable framework that couples hydrodynamic modeling of flooding driven by sea level rise and storm scenarios with site-specific building stock and census block-level demographic data. We assess the efficacy of multiple coastal adaptation strategies at reducing flooding, economic damages, and impacts to the local population. We apply this framework to evaluate a range of engineered, nature-based, and hybrid adaptation strategies for a portion of Santa Monica Bay, California. Overall, we find that dual approaches that provide protection along beaches using dunes or seawalls and along inlets using sluice gates perform best at reducing or eliminating flooding, damages, and population impacts. Adaptation strategies that include a sluice gate and partial or no protection along the beach are effective at reducing flooding around inlets but can exacerbate flooding elsewhere, leading to unintended impacts on residents. Our results also indicate trade-offs between economic and social risk-reduction priorities. The proposed framework allows for a comprehensive evaluation of coastal protection strategies across multiple objectives. Understanding how coastal adaptation strategies affect hydrodynamic, economic, and social factors at a local scale can enable more effective and equitable planning approaches.</p

Global_modern_groundwater_Gleesonetal

<p>Groundwater age transport modeling results from "The global volume and distribution of modern groundwater" by Gleeson et al. (2015) Nature Geoscience.</p><p>http://www.nature.com/doifinder/10.1038/ngeo2590</p><p><br></p><p>Gleeson, T., K. M. Befus, S. Jasechko, E. Luijendijk, and M. B. Cardenas (2015), The global volume and distribution of modern groundwater, Nat. Geosci., (November), 1–15, doi:10.1038/ngeo2590.</p

Estimating submarine groundwater discharge in a South Pacific coral reef lagoon using different radioisotope and geophysical approaches

The multi-scale and temporally variable nature of submarine groundwater discharge (SGD) has led to various ways of characterizing SGD fluxes. In this study, qualitative and quantitative methods were applied to describe SGD in a South Pacific coral reef lagoon and to determine the limitations and advantages of these ways of characterizing SGD fluxes. Synoptic and continuous monitoring of 222Rn and salinity were used over nested spatial and temporal scales and, electrical resistivity (ER) imaging was used to infer surface and groundwater salinity distributions throughout the lagoon. Moored deployments were used to estimate the lagoon residence time using ratios between radium isotopes. Spatial surveys indicated between a five and ten-fold decrease in 222Rn away from the beach face. The magnitude of the decrease was dependent on the survey scale. However, only a high resolution medium scale survey was able to detect a zone of higher 222Rn water towards the reef crest. The high 222Rn concentrations towards the reef crest corresponded to areas of higher ER and lower temperatures which indicated a point source SGD approximately 350 m off the beach. Concentrations of 222Rn and salinity during the spatial surveys and time series at each location were affected by the level of seawater recharge, the terrestrial hydraulic gradient, point source SGD, and the concentration of 222Rn laden creek water. Radioisotope measurements were used in a steady-state model, a non-steady state model and an exit point model to calculate SGD rates into the lagoon. The steady-state model resulted in the lowest estimate of 7835 m3 ∙ day− 1. The non-steady state model, using the near-shore and mid-lagoon 24 hour time series 222Rn concentrations, gave SGD volumes of 58422 m3 day− 1 and 28259 m3 day− 1, respectively. The exit point model indicated a rate of 31278 m3 day− 1. The areal-averaged SGD fluxes from the three models ranged between 0.2 and 1.8 cm day− 1. The suitability of each model for quantifying fluxes rates depended on a range of factors including the presence or absence of a dominant or local seepage zone, temporal variability, spatial heterogeneity, and the degree of mixing of low 222Rn offshore water. This study highlighted the need to employ different methods and sampling strategies to qualitatively and quantitatively characterise SGD fluxes occurring over multiple spatial and temporal scales

Global_modern_groundwater_Gleesonetal

<p>Groundwater age transport modeling results from "The global volume and distribution of modern groundwater" by Gleeson et al. (2015) Nature Geoscience.</p><p>http://www.nature.com/doifinder/10.1038/ngeo2590</p><p><br></p><p>Gleeson, T., K. M. Befus, S. Jasechko, E. Luijendijk, and M. B. Cardenas (2015), The global volume and distribution of modern groundwater, Nat. Geosci., (November), 1–15, doi:10.1038/ngeo2590.</p

Nitrogen transformations within a tropical subterranean estuary

A detailed geochemical groundwater survey was conducted within a carbonate sand subterranean estuary (STE) system on the tropical island of Rarotonga, Cook Islands, to identify N sources and transformation along the groundwater flow paths. There were two distinct sources of N to the STE: (1) local organic material which produced NH4+ in the anoxic deeper groundwater (3.5–4 m below the aquifer surface), and (2) an NO3− rich upper saline plume located in the shallow groundwater near the high tide mark of the STE. Ammonium concentrations decreased away from the organic source and the δ15N-NH4+ signature became increasingly enriched. The calculated kinetic fractionation factor for NH4+ loss was much less than that reported for nitrification in other systems, implying that mixing was important in reducing concentration. A simple fractionation/mixing model showed that NH4+ loss via nitrification increased towards the surface where it contributed up to 80% of the observed decrease in NH4+ concentration. A fast moving (~ 2 m d− 1) upper saline plume of shallow NO3− rich groundwater was a major feature of the studied STE. Denitrification was estimated to account for up to 88% of the decrease in NO3− concentration in this plume. The potential denitrification rate based on an isotope tracer addition experiment was 270 μmol L− 1 d− 1. Assuming that similar NO3− rich plumes occur all along the foreshore of the studied lagoon, we estimate that groundwater can contribute 4–15% of the daily lagoon N standing stock. We conclude that submarine groundwater discharge of shallow N rich porewater into the studied lagoon system is strongly regulated by denitrification within the STE