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218 Geothermal Energy: Renewable Energy and the Environment

Communities co-located with geothermal resources

WA MT
ND
ID
OR

SD
CA
NV
WY NE
UT
CO

AZ
NM
TX

AK

FIGUre 11.9 Map of the western United States showing locations where geothermal resources with temper-
atures greater than 50°C are located within 8 kms of a community. (Reproduced from The GeoHeat Center,
Oregon Institute of Technology. With permission.)

to allow the development of cascaded applications. Although very few of these communities have
undertaken such efforts, one community in Northern California has been remarkably successful
in developing a small resource.
The town of Canby is located in northeastern California in a region that has hot springs and warm
water at relatively shallow depths. Tests in water wells had suggested that the local geothermal gra-
dient was such that 60–70°C water would be reached at depths of about 487 m. Local hydrology
implied the aquifer at that depth and location would support a flow rate of about 9–13 l/s. Such a
resource would be very suitable for a small district heating system, with sufficient unused capacity
to allow for cascaded uses.
In 2000, a geothermal well was drilled in Canby (Figure 11.10). When the target depth was
reached, it was found that there was inadequate flow and the rock at that depth was unstable. Drilling
was continued, at increased cost, ultimately reaching a depth of about 640 m, at which point the
temperature was found to be about 85°C. The measured flow rate was just over 2 l/s. Although the
flow rate was significantly lower than expected, the temperature of the resource was adequate for
direct use purposes.
Chemical analyses of the water, however, showed mercury concentrations at about 282 ng/l,
which were well above allowable limits. After considerable effort to find remediation techniques
that would resolve the problem, it was found that a granular activated carbon (GAC) filtration system
cleaned the water to less than 1 ng/l, which was well below required standards, allowing discharge
of the return water from the district heating system to be made into a local river. The resulting dis-
charge had contaminant levels below those required for surface release to streams, and below that
of the river into which discharge took place.