Exploratory drilling to locate new water-supply wells on Cape Cod, Mas
sachusetts encountered fuel-related organic contaminants in a shallow
unconfined aquifer. However, the plume could not be traced back to the
water table and estimates of ground-water velocities were not consist
ent with plausible contaminant releases, and thus the source of contam
ination was not located. In conjunction with an expanded hydrogeologic
investigation, tritium and helium isotope analyses were performed to
delineate ground-water ages and now rates. Vertical profiles of ground
-water age were used to determine recharge rates that range from 70 to
115 cm/yr. The age of water near the leading edge of the contaminant
plume along with estimates of dispersion were used to estimate when th
e contaminant release occurred (1975 +/- 3 years). Recharge estimates,
the geometry of the aquifer, and contaminant data were used in a simp
le mass balance model to determine both the location of the contaminan
t source and the horizontal ground-water velocity (ranging from 20 +/-
12 m/y to 90 +/- 20 m/y across the site). Concurrent site investigati
ons revealed fuel floating on the water table within the area (+/- 250
m) determined by the model. The configuration of the water table and
recharge estimates were used to estimate a horizontal hydraulic conduc
tivity of 130 +/- 30 m/d. The mean hydraulic conductivity from a subse
quent 72 hr, 12 well pumping test was 100 m/d.