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depends greatly on the mode of saltwater intrusion, as well as the extent of the problem. Research investigations are therefore designed to shed light on the mode and
extent of saltwater intrusion.
There are several ideas (working hypotheses) that can explain the source of the
saltwater. One idea is that the saltwater is migrating inland from the ocean and
sounds due to sea-level rise. As the sea rises, ocean water would not only move
further inland, but the saltwater wedge beneath the coast would also migrate inland.
Although this movement would lead to saltwater sources being near agricultural
fields, the process would take years to mature because the rate of relative sea-level
rise is on the order of several millimeters per year in eastern North Carolina (see the
Kemp and Horton chapter). Another hypothesis that may explain aquifer/soil salinization is that saltwater intrusion is driven by overwash and storm surge from tropical cyclone activity. Under this process, there is an instantaneous injection of
saltwater in inundated fields that then persists in the soil or groundwater for a very
long time. Wind tide events are another mechanism by which saltwater can make its
way from surface water bodies to agricultural fields. These wind tides may cause the
water in channels to overflow into agricultural fields. The wind tides may also prolong the presence of salts adjacent to agricultural fields so that saltwater would then
have sufficient time to permeate into adjoining fields, thereby contaminating
cropland.
Land subsidence may also cause saltwater intrusion by lowering the land that is
adjacent to the sounds or ocean. The impact of this process is that the ocean/sound
level would appear to rise relative to the land surface. As a consequence, the ocean
would move inland and the saltwater wedge in the subsurface would move inland.
Processes that may cause land subsidence in eastern North Carolina may include
groundwater pumping from coastal aquifers in southern Virginia (Fig. 8.1), or the
lowering of the land surface as the Earth’s crust continues to relax due to the melting
of an ice sheet in southern Canada/northern US that occurred about 15,000 to
10,000 years ago.
For this study, we deployed devices (known as loggers) that autonomously record
water levels and specific conductivity (used here as a proxy for salinity) in surface
water and groundwater bodies (Fig. 8.4). The idea of the deployment scheme was to
place the loggers in such a way that we could capture the height and the conductivity of the water from a potential source (a large human-made canal – the AlligatorPungo River canal), along a river channel (the Alligator River, adjacent to the
Reserve; Fig. 8.1), to finally, a series of small canals in a protected coastal reserve
(Fig. 8.5). The hypothesis that we were testing for this study was that the saline
water detected in the coastal reserve originated from the large constructed canal.
The saline water then made its way to the reserve via the Alligator River and small
canals. We further hypothesized that the driver for this movement was wind tides.
To fully test these hypotheses, we collected weather data in addition to the water
quality data and water quantity data.
A. K. Manda
depends greatly on the mode of saltwater intrusion, as well as the extent of the problem. Research investigations are therefore designed to shed light on the mode and
extent of saltwater intrusion.
There are several ideas (working hypotheses) that can explain the source of the
saltwater. One idea is that the saltwater is migrating inland from the ocean and
sounds due to sea-level rise. As the sea rises, ocean water would not only move
further inland, but the saltwater wedge beneath the coast would also migrate inland.
Although this movement would lead to saltwater sources being near agricultural
fields, the process would take years to mature because the rate of relative sea-level
rise is on the order of several millimeters per year in eastern North Carolina (see the
Kemp and Horton chapter). Another hypothesis that may explain aquifer/soil salinization is that saltwater intrusion is driven by overwash and storm surge from tropical cyclone activity. Under this process, there is an instantaneous injection of
saltwater in inundated fields that then persists in the soil or groundwater for a very
long time. Wind tide events are another mechanism by which saltwater can make its
way from surface water bodies to agricultural fields. These wind tides may cause the
water in channels to overflow into agricultural fields. The wind tides may also prolong the presence of salts adjacent to agricultural fields so that saltwater would then
have sufficient time to permeate into adjoining fields, thereby contaminating
cropland.
Land subsidence may also cause saltwater intrusion by lowering the land that is
adjacent to the sounds or ocean. The impact of this process is that the ocean/sound
level would appear to rise relative to the land surface. As a consequence, the ocean
would move inland and the saltwater wedge in the subsurface would move inland.
Processes that may cause land subsidence in eastern North Carolina may include
groundwater pumping from coastal aquifers in southern Virginia (Fig. 8.1), or the
lowering of the land surface as the Earth’s crust continues to relax due to the melting
of an ice sheet in southern Canada/northern US that occurred about 15,000 to
10,000 years ago.
For this study, we deployed devices (known as loggers) that autonomously record
water levels and specific conductivity (used here as a proxy for salinity) in surface
water and groundwater bodies (Fig. 8.4). The idea of the deployment scheme was to
place the loggers in such a way that we could capture the height and the conductivity of the water from a potential source (a large human-made canal – the AlligatorPungo River canal), along a river channel (the Alligator River, adjacent to the
Reserve; Fig. 8.1), to finally, a series of small canals in a protected coastal reserve
(Fig. 8.5). The hypothesis that we were testing for this study was that the saline
water detected in the coastal reserve originated from the large constructed canal.
The saline water then made its way to the reserve via the Alligator River and small
canals. We further hypothesized that the driver for this movement was wind tides.
To fully test these hypotheses, we collected weather data in addition to the water
quality data and water quantity data.
A. K. Manda