Regional Water Availability Assessment: Delaware River Basin Active

Regional Water Availability Assessments are detailed scientific studies of water availability, conducted in medium-sized watersheds located throughout the Nation. These Integrated Water Availability Assessments, occurring in Integrated Water Science basins, evaluate water quantity and quality in both surface and groundwater, determine whether there is enough water for human and ecosystem needs, and understand how human and natural influences affect the amount of available water. Regional Water Availability Assessments address important water resource questions in the basin in ways that support local stakeholders and improve the USGS’s national water availability assessment capacity. 

The Delaware River is an interstate river, forming parts of the border for the states of New York, Pennsylvania, New Jersey, and Delaware. The Delaware River Basin (DRB) supplies fresh drinking water to over 15 million people and its uses include the support of agriculture, recreation, and endangered species habitat. Water releases from reservoirs in the upper DRB are managed by the states and the USGS Office of the Delaware River Master, and are governed by multiple criteria. Reservoir releases in the DRB have implications for both water quantity and quality. When and how water is released affects downstream conditions like the potential for drought, river temperature variations, and the location of the salt front, which is where freshwater from the Delaware River mixes with salty water from the Delaware Bay. The "Water Science and Management in the Delaware River Basin" data visualization story describes these issues and their connectivity in the DRB. 

The DRB Regional Water Availability Assessment began with pilot and planning periods (more information on the pilot can be found in the last section of this page). Phases 1 and 2 of the Regional Water Availability Assessment are described below in more detail.  

Delaware River Basin Regional Water Availability Assessment Phases 

Phase 1 of the Regional Water Availability Assessment, which formally began in 2023, will last six years and will focus on addressing a regionally relevant threat to water availability. Phase 2 of the assessment will happen in the DRB concurrently with Phase 1, will take place over two years, and will include a broader regional evaluation of water supply and demand. 

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Regional Water Availability Assessments in IWS Basins are conducted in two general Phases: 

Phase 1 - Focus topic: A focused assessment of regionally relevant threats to water availability 

Regional Assessments begin in Phase 1 by addressing regionally important water resource questions in ways that both support local stakeholders and improve national assessment capacity. A small number of focus topics are identified by the USGS Water Mission Area at the time of IWS Basin selection, guided by national model gaps and stakeholder needs in the basin. This phase of the assessment will (1) answer key science questions in the region related to the focus topic; (2) develop new process understanding and modeling capacity that fills a gap in the region and supports national models and assessments; and (3) predict future changes in water availability for humans or aquatic life, as related to the focus topic. 

In the DRB, salinity was chosen as the Regional Water Availability Assessment focus topic. Upward trends in specific conductance, total dissolved solids, sodium, and chloride were found basin wide in the pilot studies (see more below), and add to the growing body of work finding increasing saline contamination in freshwaters of the U.S. In particular, the application of deicing salt has been recognized as a dominant source of salinity to fresh waters throughout the DRB. Concentrations of sodium and chloride measured in the City of Philadelphia water supply and in surrounding municipalities have approached or exceeded some recommended health standards, and current trends suggest that these exceedances will increase in the future (Cruz et al., 2022). In addition to human health concerns, increasing salinity is associated with harmful effects to the ecological health of aquatic communities, loss of biodiversity, mobilization of metals and other contaminants, and effects on riverine nutrient cycles (Kaushal and others, 2018; Stets and others, 2020). Increasing surface and ground water salinity is a water availability concern in the DRB. The goal of Phase 1 of the DRB Regional Water Availability Assessment (focus topic phase) is to identify the drivers of increasing surface and ground water salinity and to quantify how they affect the amount of freshwater available for different uses in the past, present, and future. 

Phase 2 - Base evaluation: A regional evaluation of water supply and demand 

The Regional Assessments continue in Phase 2 with a broader evaluation—an integrated and comprehensive evaluation of factors affecting water availability in the region, inclusive of water quantity, quality, and use. This base evaluation will explore targeted future tipping points —when water no longer meets our standards for a given use — and human and ecosystem vulnerabilities in the region. A consistent approach will be used in each Regional Assessment to allow for comparing and contrasting limits to past, current, and future water availability in different parts of the Nation in more detail than will be possible using national-scale assessments. The regional base evaluation will complement the National Water Availability Assessment by answering similar water availability questions, but in more detail, and tailored toward important regional water issues. 

Delaware River Basin Pilot assessments 

The DRB pilot Regional Water Availability Assessment was launched in 2019. The pilot studies were developed to address stakeholder concerns, to establish the foundation for the DRB Regional Water Availability Assessment, and to test new approaches for broader application. 

Water quality trends are of interest to DRB stakeholders who want to identify streams that are experiencing water quality degradation and the causes of that degradation, and to track the progress and effectiveness of conservation efforts. The DRB Multisource Surface-Water-Quality Status and Trends Assessment compiled and harmonized discrete surface-water-quality data from the Water Quality Portal including data collected by the USGS, U.S. Environmental Protection Agency, Delaware River Basin Commission, and other DRB stakeholders. This fact sheet describes the data harmonization process and the resulting dataset, containing over 1.5 million data records. These data were used to generate long-term trends in water quality for 16 priority parameters. The trends showed, in general, decreases in nutrient constituents and increases in some major ions and constituents representing salinity (Shoda and Murphy, 2022). In response to these results, a follow-up investigation explored the increasing trends in specific conductance in the basin and the drivers of these trends (Rumsey and others, 2022). This work influenced the decision to pursue salinity as the focus topic for Phase 1 of the DRB Regional Water Availability Assessment. 

• Spatial patterns and seasonal timing of increasing riverine specific conductance from 1998 to 2018 suggest legacy contamination in the Delaware River Basin (journal article, 2022): Investigates increasing trends in specific conductance in the DRB from 1998-2018 and their causes.  

• River Water Quality in the Delaware River Basin— Concentrations and Trends Through 2018 (USGS fact sheet, 2023) and Water-Quality Trends in the Delaware River Basin Calculated Using Multisource Data and Two Methods for Trend Periods Ending in 2018 (USGS report, 2022): Interprets the results of surface-water-quality trends in 16 constituents at 124 sites throughout the DRB. 

• Multisource Water-Quality Trends in the Delaware River Basin webpage (website, 2020): Maps the locations with trend estimates and visually displays the trends in surface-water-quality concentrations and loads published in the 2020 data release. 

• Water-quality trends for rivers and streams in the Delaware River Basin using Weighted Regressions on Time, Discharge, and Season (WRTDS) models, Seasonal Kendall Trend (SKT) tests, and multisource data, 1978-2018 (USGS data release, 2020): Provides the results of the surface-water-quality trends analysis for 16 parameters and 124 sites in the DRB. 

• Multisource surface-water-quality data and U.S. Geological Survey streamgage match for the Delaware River Basin (USGS data release, 2019): Provides the dataset (over 1.5 million water-quality records) described in the “Pooling resources across organizations—Multisource water-quality data for the Delaware River Basin” fact sheet. 

• Pooling resources across organizations—Multisource water-quality data for the Delaware River Basin (USGS fact sheet, 2020): Describes the process used to retrieve multisource surface-water-quality data for the DRB and prepare those data for analysis, and provides a preliminary assessment of the resulting dataset. 

• A historical look at changing water quality in the Delaware River Basin (USGS fact sheet, 2020): Summarizes previously conducted USGS studies that contain sites in the DRB and specifically highlights findings from the basin. 

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Harmful algal blooms (HABs) pose threats to humans and aquatic ecosystems by creating aquatic conditions which can produce toxins, reduce dissolved oxygen concentrations, and alter aquatic food webs. The DRB Algal Assessment focused on characterizing algal communities and the likelihood of nuisance/harmful algal bloom occurrence within four sub-watersheds of the DRB in New Jersey by measuring algal biomass, the presence of algal groups responsible for toxins and nuisance blooms, nutrients, and other parameters. This project also explored novel sampling techniques including Solid Phase Adsorption Toxin Tracking (SPATT) samplers and, in collaboration with the USGS Leetown Science Center, new sampling methods and processing techniques for eDNA. Using the DRB as a test bed for new technology and sampling approaches allows the USGS to advance capabilities while also benefiting basin stakeholders by providing specific information for the study watersheds. 

Understanding baseflow, which is groundwater that seeps to the surface and sustains many streams and rivers, and how much, when, and where it is present, is necessary to assess the vulnerability and potential response of aquatic systems to changes in environmental conditions, including increasing salinity. The DRB Assessment of Baseflow Contributions to Streamflow, which generated estimates of daily and monthly baseflow in the DRB, improves our understanding of how and when groundwater is sustaining surface-water supply.  

 

• Monthly estimates of natural baseflow for 15,866 stream reaches, defined by the National Hydrography Dataset Plus Version 2.0 (NHDPlusV2), in the Delaware River Basin for the period 1950-2015 (USGS data release, 2021): Provides understanding of the temporal and spatial variability of baseflow in the basin. 

• Daily-timestep and monthly-timestep estimates of baseflow at 49 reference stream gages located within 25 miles of the Delaware River basin watershed boundary for the years 1950 through 2015 (USGS data release, 2020): Provides an initial understanding and was used to develop the dataset for all stream reaches in the DRB.

The Simulation and Analysis of Groundwater Dynamics component of the DRB pilot generated estimates of the age of DRB groundwater that is delivered to streams, contributed to the development of a groundwater flow model, and developed a model to predict the depth to regolith, the geologic layer above bedrock, in the basin. Each of these studies enhances our ability accurately capture conditions in the shallow subsurface of the basin, thus improving our ability to assess surface water and groundwater as a single resource, and predict the potential effects of climate variability on groundwater quantity and quality. 

Understanding drought history in the DRB can contextualize current conditions and help identify drivers of drought in the future. A characterization of the long-term history of the magnitude, frequency, duration, and severity of droughts in the DRB was conducted and published in the article Hydro-climatic drought in the Delaware River Basin. This work used both the instrumented record and paleo-reconstructions to determine climatic drivers of drought.