ADVANCES IN PASSIVE HEAD SPACE SAMPLING AND APPLICATION IN LONG-SCREENED WELLS

            Passive headspace samplers have been used for a variety of dissolved gas work including noble gases, but are generally limited to shallow depths (<50 m) and require a measurement of total dissolved gas pressure to compute dissolved concentrations.  We developed a high-pressure passive sampler (HPPS) that preserves the in-situ gas pressure using a simple down-hole, hydraulically actuated valve, with a theoretical depth limit of about 1000m.  These samplers allow discreet dissolved gas sampling long-screened well.  Using the HPPS, we collected nine samples spaced 12m apart over a long-screened interval from 262 to 166m below land surface in an exploration well near Tyrsting Denmark.  Sampling of three nested, short-screened wells at depths of 30-50m, 70-80m, and 100-110m completed the vertical profile at the site.  Water samples in conventional copper tubes were also collected using a sophisticated discrete depth sampling system that employed 3 pumps and a computer control system.  Measurements of He isotopes, Ne, Ar, Kr, and Xe were made using both methods.  Tritium and helium measurements loosely constrain the bomb peak between 50 and 100m with a maximum of 34.6 TU of tritiogenic ³He and an R/Ra of 2.2 at 75m.  Assuming bomb peak water recharged in 1964 is now at a depth of 75m and a porosity of  0.3 we calculate a recharge rate of 550mm/yr - about 65% of the annual precipitation at the site.  He profiles show an increased terragenic ⁴He component at the maximum depth with an R/R_a of .597 using the HPPS; however, there is little measurable terragenic ⁴He above 238m in depth and no evidence for an allochthonous upward ⁴He flux.  Virtually no vertical variations were observed using the computer controlled pumping system, presumably due to by-passing in the gravel pack.  Using the passive sampler data, we can constrain internal ⁴He production. Assuming a Vogel model for age stratification with depth and the recharge given above, we estimate an age of 340 years for water at 238m.  Given this age we can constrain the internal ⁴He production in the aquifer to less than 1280 atoms/m³/s. These preliminary results illustrate the potential to obtain vertical profiles of noble gas concentrations in deep boreholes, which has significant implications in recharge estimation, paleoclimate reconstruction and subsurface helium production studies.