Ground Water Availability and Resource Protection

1
Ground Water AvailabilityandResource Protection

• June 2007

2
General Water Supply Issues

• Sole Source Aquifer - no significant fresh water
  streams or rivers
• Limited resource - recharge occurs in a Spine
  Area
• Fresh ground water is restricted to depths less
  than 350 feet
• Multiple contamination threats - to the water
  table from land use activities and to the
  confined aquifers from over pumping
• Most likely cause for a loss of fresh ground
  water is salt water intrusion due to over pumping

3
What is an Aquifer

• Any coarse grained material (sand, gravel) that
  can supply sufficient water for a beneficial use
• A confined aquifer is one that is under pressure
  (the water surface is higher than the physical
  top of the aquifer)
• An unconfined or water table aquifer is the
  closest to the surface and is not under pressure

What is a Confining Unit

• Any fine grained material (silt, clay) that can
  significantly restrict vertical movement of
  groundwater such that the underlying groundwater
  is under pressure.

4
Schematic Cross-Section of Freshwater Lens
WEST
Water Table Surface
Confining Unit
Freshwater Aquifer
Saltwater Aquifer
Estimated Recharge to Water Table Aquifer 625
MGD approx Estimated Recharge to
Yorktown-Eastover Aquifer 8 MGD approx
5
Water Budget for the Water Table Aquifer
Total Estimated Recharge to Water Table Aquifer
625 MGD
6
Permitted Groundwater Withdrawals
7
Total Water Budget in the Confined Aquifer
Public/Industrial pumping domestic pumping Is
equal to Water Leaking through upper confining
unit water released from storage
8
Groundwater Level Trends Near Perdue
9
Recharge to Yorktown-Eastover is Limited

• USGS collected water samples collected from wells
  and age dated them
• Much of the water table aquifer was less than
  50-years old.
• In contrast, average age for water samples from
  the upper Yorktown-Eastover exceeds 1,000 years

10
Rate of Flow Through Upper Confining Unit
11
Schematic Cross-Section of Freshwater/Saltwater
Interface
12
Increasing Chloride Trends Over TimeTown of
Chincoteague Well
13
Cape Charles Chloride Levels Near Production
Wells
14
Chloride Increase With DepthUpshur Neck Seaside
Example
Fresh Ground Water
Brackish Ground Water
15
Available Water Evaluations

• Previous Used Tools
• Empirical Observations
• Analytical models / calculations
• Numeric Models USGS SHARP Model EPA FEMWATER
  models
• Current Tool
• USGS Eastern Shore Model (SEAWAT)

16
USGS SEAWAT Model

• Beta Version
• Major Enhancements
• Increased resolution
• minimum horizontal size 1,000x1,000 Feet from
  original 1-mile
• Vertical layers increased from original 4 to 46.
• Functional Water Table
• Increased lateral extent into Maryland
• Ability to simulate gradational increase in
  saltwater
• Calibrated from 1920 through 2003 at 20
  observation points

17
Three Dimensional Grid