Delaware is a very well-monitored state. State and federal agencies and several environmental organizations collect and analyze water quality samples statewide. Because so much data is available, we know that many of our waters are considered impaired due to high levels of nutrients like nitrogen and phosphorus, which contribute to algal blooms and large fluctuations in dissolved oxygen levels. These conditions negatively impact the ability of aquatic life to survive and thrive in some locations.

Surface Water Trends

Surface waters are streams, ponds, lakes, rivers, estuaries, and oceans.

In 2025, the US Geological Survey released a summary of water quality trends for surface water stations across the non-tidal portions of the Chesapeake Bay watershed for the ten years of 2014-2023. Approximately one-third of the state of Delaware drains to the Chesapeake Bay. This monitoring network includes eight stations on Delmarva where the streams originate in Delaware. This report found that for total nitrogen, four stations (50%) were improving (concentrations decreased over time), three stations (38%) were degrading (concentrations increased over time), with one station (12%) having no statistically significant trend in either direction. Within the last 10 years, the number of stations with improving nitrogen trends has increased. For total phosphorus, all of the stations (100%) were degrading.

In 2025, DNREC released its Combined Watershed Assessment Report (305(b)) and Determination for the Clean Water Act Section 303(d) List of Waters Needing TMDLs, also known as the “Integrated Report.” This report includes a long-term trend analysis for 11 surface freshwater stations from across the entire state over the period of 1979 through 2019, 40 years. This analysis found that for total nitrogen, six stations (55%) were improving, three stations (27%) were degrading, and two stations (18%) had no trend. For total phosphorus, five stations (46%) were improving, three stations (27%) were degrading, and three stations (27%) had no trend.

Thus, depending on location, the period under consideration, and the parameter of interest, water quality in Delaware and the region is both improving and degrading.

The Integrated Report also assesses surface water quality conditions for 134 stations between 2018 and 2022. During this time, 37 stations (27%) exceeded the three milligram per liter (mg/L) threshold for total nitrogen, while only one station exceeded the total phosphorus threshold of 0.2 mg/L (<1%). Thus, while trend analyses may indicate that water quality conditions are improving at some stations, more than a quarter of Delaware’s monitoring stations have excess nitrogen.

Groundwater Trends

Most of the water in Delaware streams originates as groundwater. When it rains on land, water percolates through our sandy soils and enters the surface aquifer, becoming groundwater. As that water moves downward through the soil, it also picks up dissolved pollutants, like nitrogen. But, it can take years to several decades for groundwater to move under and through a landscape to where it finally discharges to a surface water body. The quality of our surface waters, especially when made up primarily of discharging groundwater, reflects a combination of what happened on the land's surface in the last 50 years. This is why monitoring the quality of our groundwater is essential, since it will give us a sense of how our more recent management actions will impact stream water quality in the future.

The US Geological Survey has monitored groundwater quality in Delaware and across the region numerous times over the years. The National Water-Quality Assessment Program monitored groundwater quality in the 1990s, early 2000s, and in 2012. With this data set, when comparing the early 2000s data to 2012, the range of nitrogen concentrations was about the same, but the median concentration appeared to decrease from about 9 mg/L to 6 mg/L nitrate as nitrogen. This decrease possibly reflects the effectiveness of nutrient management practices employed in our state and region since the late 1990s. Unfortunately, the sample set was insufficient to consider this decrease statistically significant (J. Denver, oral communication). But, this apparent decline was promising and prompted interest in a thorough assessment of shallow groundwater conditions in our region to credit nutrient management for the benefit of water quality.

Working with the Delaware Department of Agriculture, the USGS established a network of shallow groundwater wells in agricultural areas across the Delaware coastal plain. This network of over 40 wells was sampled in 2014 and 2019. Nitrate concentrations ranged from 0.45 mg/L to 30.7 mg/L, with a median of 5.09 mg/L in 2019. This median was slightly higher than the median in 2014, though not statistically significant. This monitoring effort suggests little change in nutrient concentrations from 2014 to 2019.

Travel-time of Nitrate Through Groundwater to Streams

As it was stated earlier, it takes a long time for groundwater to move and we may need to wait more than a decade to see improvements in our stream samples. But, understanding this process is vitally important to communicating our nutrient management efforts and progress towards improved water quality.

The image below depicts a theoretical groundwater flow path beneath an agricultural setting and the age and nitrate concentration of that water along the path (Figure 1). The lighter shade of green suggests lower nitrate entering the groundwater aquifer due to more recent use of nutrient management practices. The darker shades of green correspond with older groundwater with higher concentrations of nitrate, which entered the aquifer before extensive use of nutrient management practices. Ideally, continued use of nutrient management over time will lower nitrate levels along the entire groundwater flow path and in the streams, too. Image courtesy of Judy Denver, USGS (retired).

Figure 1. Visual representation of the travel time of Nitrate through Groundwater to Streams

About the Author

Jennifer Volk (corresponding author), Associate Director, University of Delaware Cooperative Extension, Dover, DE (jennvolk@udel.edu)

About this Publication

Original Publication Date: March 2014

Revision date(s): October 2025

Peer Reviewers

Judy Denver, U.S. Geological Survey, Dover, DE (2014)

Bhanu Paudel, Delaware Department of Natural Resources and Environmental Control, Dover, DE (2025)

Alex Soroka, U.S. Geological Survey, Baltimore, MD (2025)

References

2026 State of Delaware Ambient Surface Water Quality Monitoring Program, 2025, documents.dnrec.delaware.gov/Watershed/Assessment/Monitoring/FY2024-Water-Quality-Monitoring-Plan.pdf

Denver, Judith M, et al. “USGS.” Water Quality in the Northern Atlantic Coastal Plain Surficial Aquifer System, Delaware, Maryland, New Jersey, New York, North Carolina, and Virginia, 1988–2009, 2014, pubs.usgs.gov/circ/1353/pdf/circ1353optimize.pdf

“Integrated Report: 305(B) Report and 303(D) List.” DNREC, 30 July 2025, dnrec.delaware.gov/watershed-stewardship/assessment/reports/

Mason, Christopher, et al. “Summary of Nitrogen, Phosphorus, and Suspended- ...” Summary of Nitrogen, Phosphorus, and Suspended-Sediment Loads and Trends Measured at the Chesapeake Bay Nontidal Network Stations for Water Years 2014–2023, 2025, d9-wret.s3.us-west-2.amazonaws.com/assets/palladium/production/s3fs-public/media/files/NTN-Load-and-Trend-Summary-2023.pdf

Soroka, Alexander M, et al. “USGS.” Comparison of Water Quality in Shallow Groundwater Near Agricultural Areas in the Delaware Coastal Plain, 2014 and 2019, 2024, pubs.usgs.gov/sir/2024/5111/sir20245111.pdf

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