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Around 40% of the world's population depends on coastal aquifers for freshwater supply but natural and anthropogenic drivers threaten groundwater availability. Of these drivers, saltwater intrusion (SWI) is one of the most critical and is increasingly affecting coastal areas worldwide. Interest in coastal aquifers has significantly increased, as demonstrated by the growing number of publications in which researchers describe various approaches to illuminate the importance of coastal aquifers, specifically with regard to SWI. The state of research and knowledge of the coastal SWI issue has been reviewed herein. The review includes a discussion of select geophysical and field methods and tools which can inform the numerical modeling of coastal aquifers. MODFLOW was identified as the most often used numerical modeling platform. Further, while many research sites, particularly in the United States, were identified where field studies and geophysical methods, mostly geoelectric ones, added important value to the numerical modeling of the SWI process in the coastal zone, in some regions of the world, data scarcity was identified as the main challenge. Overall, numerical modeling, combined with geophysical methods, is a valuable tool for studying SWI and managing coastal water resources. [ABSTRACT FROM AUTHOR]

The coastal zone, which is the interface between land and sea, is hydrodynamically very active due to the complex interactions of various hydrological controls and variable‐density fluids. These forces vary over time, resulting in a state of dynamic equilibrium in the system. The major hydrological processes in coastal aquifer systems are salt water intrusion and submarine groundwater discharge, which are interdependent. Monitoring these complex processes is crucial for sustainable coastal zone management but poses a significant research challenge. In this study, we demonstrate the effectiveness of non‐invasive geophysical techniques, specifically the time‐lapse electrical resistivity imaging method, in conjunction with groundwater monitoring, for monitoring coastal groundwater dynamics in an unconfined aquifer at varying time scales and hydrogeological settings present at formerly glaciated sites worldwide. We generated two‐dimensional baseline salt water intrusion maps for the test site, located on the coast of Rhode Island, USA. The time‐lapse electrical resistivity survey method enables the rapid estimation of fresh groundwater discharge. Our approach offers insight into the mechanisms and seasonably variable salt water–freshwater interactions in unconfined heterogeneous aquifers. Although the results are site‐specific, their implications are broad and may stimulate other studies related to sea to land pollution (sea water intrusion) and land to sea pollution (groundwater discharge) in heterogeneous coastal aquifer settings. [ABSTRACT FROM AUTHOR]

The general practice of rainfall-runoff model development towards physically based and spatially explicit representations of hydrological processes is data-intensive and computationally expensive. Physically based models such as the Soil Water Assessment tool (SWAT) demand spatio-temporal data and expert knowledge. Also, the difficulty and complexity is compounded in the smaller watershed due to data constraint and models' inability to generalize hydrologic processes. Data-driven models can bridge this gap with their mathematical formulation. Long Short-Term Memory (LSTM) is a data-driven model with Recurrent Neural Network (RNN) architecture, which is better suited to solve time series problems. Studies have shown that LSTM models have competitive performance in watershed hydrology studies. In this study, a comparative analysis of SWAT and LSTM models in the Cork Brook watershed shows that results from LSTM were competitive to SWAT in flow prediction with NSE of 0.6 against 0.63, respectively, given the limited availability of data. LSTM models do not overestimate the high flows like SWAT. However, both these models struggle with low values estimation. Although interpretability, explainability, and use of models across different datasets or events outside of the training data may be challenging, LSTM models are robust and efficient. [ABSTRACT FROM AUTHOR]

Constructed floating wetlands is viable alternative for the treatment of stormwater, combined stormwater-sewer overflow, sewage and water supply reservoirs, among others. The use of this technology also allows to enhance the habitat, and improve aesthetics to the treatment facility. In brief, the constructed floating wetlands island treatment mechanism is a combination of several components and physico-chemical processes that mimic natural bioremediation. Plant roots play a major role in treatment processes within constructed floating wetland island since the water passes directly through root system underneath the floating mat. Pathways for contaminant removal/retention in floating wetland island are: release of extracellular enzymes, development of biofilms, flocculation of suspended matter, and plant uptake. This study summarizes the findings of four monitoring studies and emphasizes on the field studies that monitored how pond contaminants responded to the floating wetlands through extensive review of existing literature.

Nitrogenous disinfection by-products (N-DBPs) are emerging by-products that may be present in drinking water as by-products of water treatment plant (WTP) operations. Nitrosamines are N-DBPs that form by reaction of chloramine with certain organic nitrogen-containing compounds; however, the exact processes and environments in which nitrosamines form are still not well understood. Organic nitrogen precursors react within the WTP and distribution system, forming the toxic by-products during chloramination, or while in distribution. To best control the formation potential of nitrosamines, precursors must be removed from source water prior to chloramine disinfection. These nitrosamine forming precursors are abundant in source waters worldwide, presenting a need for further study of the mechanisms that reduce the formation potential of nitrosamines in chloramination WTPs.

Riparian zones are often used as best management practices due to their ability to remove nitrate (NO3−) from subsurface flow. Research suggests that beyond local biogeochemical controls, the impact of riparian zones on nitrogen removal and other functions, such as phosphorus dynamics and greenhouse gas emissions, largely depends on land-use/land-cover, hydrogeomorphology, and weather. In this study, we therefore present RZ-TRADEOFF, a novel and easily applicable model that connects multiple riparian functions and characteristics (NO3− and phosphate (PO43−), concentration and removal in subsurface flow, total phosphorus (TP) removal in overland flow, nitrous oxide (N2O), methane (CH4), and carbon dioxide (CO2) emissions, water table) to landscape hydrogeomorphic characteristics, weather, and land-cover/land-use. RZ-TRADEOFF was developed with data from past studies and digital databases, and validated with data collected from the literature. Three functions (water table, PO43− and CO2) were observed to be significantly influenced by climate/weather, while the others were primarily influenced by hydrogeomorphology and land use. The percent bias and normalized root mean square error respectively were −3.35% and 0.28 for water table, 16.00% and 0.34 for NO3− concentration, −7.83% and 20.82 for NO3− removal, 6.64% and 0.35 for PO43− concentration, 2.55% and 0.17 for TP removal, 40.33% and 0.23 for N2O, 72.68% and 0.18 for CH4, and −34.98% and 0.91 for CO2. From a management standpoint, RZ-TRADEOFF significantly advances our ability to predict multiple water and air quality riparian functions using easily accessible data over large areas of the landscape due to its scalability. [ABSTRACT FROM AUTHOR]

Campbell A, Pradhanang SM, Kouhi Anbaran S, Sargent J, Palmer Z, Audette M. 2017. Assessing the impact of urbanization on flood risk and severity for the Pawtuxet watershed, Rhode Island. Lake Reserv Manage. 34:74-87.This study models the Pawtuxet River watershed with the Soil and Water Assessment Tool (SWAT) and high spell analysis to understand the impacts of land use and management on flood events. The watershed is in central Rhode Island and contains the Scituate Reservoir and the southern suburbs of Providence. The area is divided between urban development along Narragansett Bay and forested land cover around the Scituate Reservoir. The model was calibrated and validated to a daily time step. The calibration data was for the entirety of 2010 including a flood event in March and early April, which caused significant damage to the region. The calibrated model had a Nash-Sutcliffe efficiency (NSE) of 0.69. The validation had an NSE of 0.65. The model was run with 3 land use/land cover scenarios: the 2011 statewide LULC as a baseline, increased urbanization around Narragansett Bay, and increased urbanization surrounding the reservoir. In the 2011 model, urban land cover accounted for only 13.49% of the study area. Simulated sub-daily flow for the watershed, at a 5 min time step, was used to understand the potential impact of urbanization on flood events. Each year from 2010 to 2013 was analyzed with high spell analysis; the average high spell volume in 2010 was nearly double all other years. The scenarios with increased urban land use saw an increase in the mean volume of high spells, however there was not an increase in the 2010 floods peak. [ABSTRACT FROM AUTHOR]

Climate studies have suggested that inland stream temperatures and average streamflows will increase over the next century in New England, thereby putting aquatic species sustained by coldwater habitats at risk. This study uses the Soil and Water Assessment Tool (SWAT) to simulate historical streamflow and stream temperatures within three forested, baseflow-driven watersheds in Rhode Island, USA followed by simulations of future climate scenarios for comparison. Low greenhouse gas emission scenarios are based on the 2007 International Panel on Climate Change Special Report on Emissions Scenarios (SRES) B1 scenario and the high emissions are based on the SRES A1fi scenario. The output data are analyzed to identify daily occurrences where brook trout (Salvelinus fontinalis) are exposed to stressful events, defined herein as any day where Q25 or Q75 flows occur simultaneously with stream temperatures exceeding 21 °C. Results indicate that under both high- and low-emission greenhouse gas scenarios, coldwater fish species such as brook trout will be increasingly exposed to stressful events. The percent chance of stressful event occurrence increased by an average of 6.5% under low-emission scenarios and by 14.2% under high-emission scenarios relative to the historical simulations. [ABSTRACT FROM AUTHOR]

It has become increasingly important to recognize historical water quality trends so that the future impacts of climate change may be better understood. Climate studies have suggested that inland stream temperatures and average streamflow will increase over the next century in New England, thereby putting aquatic species sustained by coldwater habitats at risk. In this study we evaluated two different approaches for modeling historical streamflow and stream temperature in a Rhode Island, USA, watershed with the Soil andWater Assessment Tool (SWAT), using (i) original SWAT and (ii) SWAT plus a hydroclimatological model component that considers both hydrological inputs and air temperature. Based on daily calibration results with six years of measured streamflow and four years of stream temperature data, we examined occurrences of stressful conditions for brook trout (Salvelinus fontinalis) using the hydroclimatological model. SWAT with the hydroclimatological component improved modestly during calibration (NSE of 0.93, R² of 0.95) compared to the original SWAT (NSE of 0.83, R² of 0.93). Between 1980-2009, the number of stressful events, a moment in time where high or low flows occur simultaneously with stream temperatures exceeding 21 °C, increased by 55% and average streamflow increased by 60%. This study supports using the hydroclimatological SWAT component and provides an example method for assessing stressful conditions in southern New England's coldwater habitats. [ABSTRACT FROM AUTHOR]

Nonpoint source nitrogen pollution is difficult to effectively model in groundwater systems. This study aims to elucidate anthropogenic nonpoint source pollution discharging into Potowomut Pond and ultimately Narragansett Bay. Hydrologic modeling with Soil and Water Assessment Tool (SWAT) and SWAT Calibration and Uncertainty Program (SWAT-CUP) was used to simulate streamflow and nitrogen levels in the Hunt River with and without onsite wastewater treatment systems (OWTS). The objective of this study was to determine how input of OWTS data impacts nitrogen loading into the Hunt River Watershed in Rhode Island, USA. The model was simulated from 2006 to 2014, calibrated from 2007 to 2011 and validated from 2012 to 2014. Observed streamflow data was sourced from a US Geological Survey gauge and nitrogen loading data from University of Rhode Island Watershed Watch (URIWW). From the results, adding OWTS data to the SWAT simulation produced a better calibration and validation fit for total fit (Nash-Sutcliffe Efficiency (NSE) = 0.50 calibration, 0.78 validation) when compared with SWAT simulation without OWTS data (NSE = -1.3 calibration, -6.95) validation. [ABSTRACT FROM AUTHOR]