Groundwater
characteristics:
Bedrock aquifers
Bedrock wells in the San Juans are completed “open hole” which is the practice of installing a 6-inch diameter steel casing in the first few feet of bedrock, sealing it with cement and/or bentonite clay, and then drilling the remaining bore hole without additional casing. Generally, several water-bearing fractures are encountered during drilling, making up a total yield for the well.
Bedrock wells are generally deep; 82% are drilled below sea level. Water levels in most of these wells are within zero to 50-feet of surface, indicating that the water is flowing into these wells from higher elevations. Well yield based on driller’s logs indicate that these wells range from zero to 100 gallons/minute, with 51% yielding two gallons-per-minute or less. This well yield information is based on a brief test at the time of drilling. Well yield information from longer duration pump tests tend to confirm the lower yield rates from driller’s logs and show that the higher rates are not sustainable.
Recharge
potential in bedrock is
limited due to its impervious nature and the fact that most bedrock
terrain is
steep. USGS estimates for recharge in
bedrock are 10- to 50- percent of that in areas with glacial deposits.
Water-bearing
zones in fractured
rock are susceptible to dewatering from pumping, particularly during
the dry
season. Fractures can be conduits to
contamination from surface water and seawater intrusion.
Upper fractures can fill with water during
the rainy season and convey surface water directly into the well. Bacterial contamination and the presence of
tannins from organic material is common in these wells in late fall and
early
winter (SJCHCS). These wells are also
subject to natural contaminants such as barium, arsenic, fluoride, and
sodium,
which pose risks to human health, and to iron, manganese and hardness
(CaCO3)
which cause ascetic and functional problems, such as odors, staining,
and
mineral deposits.
Bedrock
wells account for 74% of
the wells documented in the county. Of
these, approximately 900 residences, mostly on
Aquifers
in glacial deposits in
Yield
from glacial wells is much
higher than bedrock, although
Glacial
wells account for
approximately 26% of the wells developed in the county.
Of these, an estimated 75% are in areas at
risk for seawater intrusion.
Seawater
intrusion is the
replacement of fresh ground water with seawater. This
process occurs when freshwater, which is
lighter than seawater and floats as lens on the underlying seawater, is
withdrawn to the point that seawater is either drawn up toward the
bottom of a
pumping well (upconing) or moves laterally inland (lateral intrusion)
because
of depletion of the volume of the fresh water lens.
Seawater intrusion is
a dynamic
process that varies with seasonal recharge, tidal changes, changes in
withdrawal rates, and, to some extent, barometric pressure. Chloride is a stable ion found in seawater
that is easily measured in freshwater and commonly used as an indicator
of
seawater intrusion. State policy uses
100 mg/L chloride concentration as a threshold for risk of seawater
intrusion. Studies in
_______________________________________________________________________
•
Estimated
background chloride concentrations
range from 10 to 100 mg/L, whereas ground water containing more than 160 mg/L is strongly suspected of being
contaminated by seawater. Because a zone of diffusion exists where
freshwater
and seawater mix, the use of a single concentration value
. . . albeit convenient, is not entirely
realistic.
•
Median
change in chloride concentrations in
samplings between April and September was zero for wells with less than
160
mg/L and 32 for samples with more than 160 mg/L.
–
Source:
USGS WRIR 83-4019, Whiteman, et al,
1983
Wells
in the shoreline are often
drilled below sea level, which increases the risk of penetrating the
fresh/seawater interface. Most of the
shoreline development in the county is served by individual and
community
wells. Near the shoreline the freshwater
lens is narrowest, most vulnerable to lateral intrusion, and also to
the impact
of upconing in neighboring wells.
Several
shoreline areas in the
county are affected by seawater intrusion, however, whether this is due
to
local upconing or a more extensive depletion that is resulting in
lateral
intrusion is not known. Some parts of
the county, particularly geographic landforms such as peninsulas and
isthmuses
with limited acreage, do not have sufficient aquifer recharge available
to
supply current and future users.
Recharge:
Recharge
is freshwater that
replenishes aquifers on an annual basis.
In
The
USGS study included two
years of stream-flow and weather station data collection, including
interception loss under the tree canopy.
Rainfall was analyzed for chloride in order to use the track of
this
stable ion to confirm the main analytical method (deep percolation
model or
DPM) and also to look at chloride deposition in rainfall (compared to
seawater
intrusion). Recharge conditions were
then modeled using a daily measurements for precipitation, thoughfall,
solar
shortwave radiation, air temperature, and stream discharge data applied
to
soil, vegetation, and landcover conditions.
(See Table x.1, page x)
Comparing
recharge estimates to
residential development identifies several areas of the county where
demand for
water exceeds available recharge.
Analysis of well yield information from drilling logs tends to
confirm
low yields in these areas. The extent of
this problem in the county is probably most apparent in documentation
of the
amount of water hauling that occurs during the summer months when
demand is
highest, recharge lowest, and wells fail.
In 2003, 10%, xx gallons, of the water the Town of Friday Harbor
produces annually was sold for hauled water to supplement failed wells
serving
individual and community systems on the island.
In response to a survey by Health and Community Services, water
haulers
on
Some bedrock areas of the county have very little capacity for storage or transmission of groundwater. These areas are generally located inland in rural designated areas, although some coastal areas are also affected. These areas tend to have high, steep elevations, with limited soil cover over the bedrock. They have low recharge estimates and low average well yield based on drilling log data. It is generally assumed that the fractures that supply these wells have little interconnectedness, and that withdrawal from one well does not affect a neighboring well.
In glacial deposits low yield wells are generally in areas that are predominately till, which is composed of a compressed mixture of materials, including fine-grained silts and clay, that act as aquitards and restrict the flow of water.
Limited aquifer capacity along shoreline areas is also subject to depletion, but rather than well failure, will draw seawater into the aquifer.
The main concern for groundwater management is in areas with low yield wells and land use designations of less than five acres, where the overall capacity for recharge may not be sufficient for the demand for water.
There
are several
naturally-occurring contaminants in
Arsenic,
barium, fluoride, and
sodium are present in many areas in the county at levels that present a
health
risk. Exposure to arsenic over time
increases the risk of cancer; barium can cause high blood pressure and
changes
to the heart; fluoride in high doses can cause brittle bones and at
high levels
poses a risk to children with developing teeth, particularly if
combined with
fluoride supplements (which in low doses helps to form strong teeth). The county will not issue a building permit
for use of a well with primary contaminants over the maximum
contaminant
level. Continued data collection,
mapping, and public education are the main management recommendations
for these
conditions.
The
fractures of bedrock wells,
and the porous conditions of some glacial sediments provide an avenue
for
surface contaminants to enter groundwater.
Fractures are a particular concern, since they provide no
filtration
benefits. Wells in some formations are
contaminated every winter by surface water that finds its way into
upper
fracture zones. In some cases, this
surface water picks up bacteria and other pathogens that contaminate
the
wells. Currently, state code does not
require on-site septic systems to be designed in a way that prevents
downward
migration of contaminants, such as nitrates.
Where
wells
and on-site septic systems are developed on small lots the risk of
contamination is compounded. Many small
lots were created in
·
The
location, capacity, and quality of the county’s aquifers are not known. Recharge estimates, well logs, and limited
water quality data are the only sources of information available about
groundwater conditions in
·
Allocation
of water rights for groundwater sources exceeds available recharge in
many
parts of the county. In addition, there
are several areas with potential for high-density new development and
infill
that do not have adequate groundwater supplies.
·
The
risk of seawater intrusion has been documented in the county, but the
extent
and timing of this risk is not known.
The state and county have the responsibility to manage and
protect the
groundwater resource, but at this time there is no working agreement or
plan
for management.
·
In
order to supply water in areas with depleted groundwater sources,
coordination
and cooperation with the county’s community water systems is needed. At this time, these systems do not have the
capacity, or desire, to serve additional customers.
·
Rural
build-out in areas with low-yield wells and naturally occurring
contamination
will require education and on-going documentation of conditions.
XXmaps
of contaminants
…maps:
chloride map with radii, location of A wells with charts
…graphic:
intruded well
…Maps:
geology, well yield, recharge and yield/population; recharge/population
…recharge
maps of each island.