Where Does SWI Occur?

Where Does Saltwater Intrusion Occur in Delmarva Agricultural Fields?

Salt-affected soils are not evenly distributed across coastal agricultural fields. On the Delmarva Peninsula, salinity is often greatest in low-elevation areas near tidal wetlands, creeks, and drainage ditches, where fields are more exposed to saltwater flooding and shallow saline groundwater.

Within a field, salt concentrations can change over relatively short distances. Areas closest to tidal water may have the greatest salt accumulation, while concentrations generally decrease as elevation increases and flooding becomes less frequent.

Wetland edges

Fields that border tidal wetlands are particularly vulnerable to salt accumulation. Storm surge, high tides, and other flooding events can bring saline or brackish water onto adjacent agricultural soils. As the water drains or evaporates, salts can remain in the soil.

These low-lying areas may also have limited opportunities for leaching, particularly where the groundwater table is shallow or continued saltwater exposure occurs. As a result, salt can accumulate over time and create patches where crop growth is reduced or conventional production becomes difficult.

Field ditches

Drainage ditches are another common location for salt-affected soils. Ditches connect agricultural fields to tidal waters and can provide pathways for saline water to move inland during flooding events.

Our field sampling has found saline and saline-sodic soils along some ditch edges, with salt-affected areas extending beyond the ditch and into adjacent portions of fields. The extent of this effect varies with elevation, ditch connection to tidal waters, soil properties, and flooding history.

EC of three different fields compared to elevation.

Elevation matters

Elevation is an important factor in determining where salt accumulates. Lower portions of coastal fields are more likely to experience flooding and shallow saline groundwater, while higher portions are generally less exposed. Field observations show that soil salinity can decrease rapidly over relatively small changes in elevation.

Salt-affected soil is not always obvious from the field surface. Electrical conductivity mapping and soil sampling have identified areas of elevated salinity extending from tidal wetlands and drainage ditches into otherwise productive fields.

Flooding and seasonal changes

Salt concentrations can also change throughout the year. Flooding events can introduce additional salt to the soil, while rainfall and drainage can move soluble salts downward and reduce concentrations in the root zone.

Our monitoring has shown increases in soil electrical conductivity following major flooding events, followed by declines during periods of greater rainfall and leaching. The timing and magnitude of these changes vary among fields and years.

What does this mean for a field?

The location and severity of salt accumulation depend on several interacting factors:

Elevation and distance from tidal water
Flooding frequency and duration
Drainage and groundwater conditions
Soil texture and water movement
Connection to tidal creeks and drainage ditches
The amount and type of salt entering the field

Because these factors vary within a field, saltwater impacts can occur as patches or gradients rather than a uniform field-wide problem.

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