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P.G. Cook

Publications and source records attributed to P.G. Cook.

9 recordsLinked to original sources
USGS2010

Chlorofluorocarbons as tracers of groundwater transport processes in a shallow, silty sand aquifer

Detailed depth profiles of Chlorofluorocarbons CFC-11(CFCl 3 (, CFC-12 (CF 2 Cl 2 ) and CFC-113 (C 2 F 3 Cl 3 ) have been obtained from a well-characterized field site in central Ontario. Aquifer materials comprise predominantly silty sands, with a mean organic carbon content of 0.03%. Nearly one-dimensional flow exists at this site, and the vertical migration of a well-defined 3 H peak has been tracked through time. Detailed vertical sampling has allowed CFC tracer velocities to be estimated to within 10%. Comparison with 3 H profiles enables estimation of chlorofluorocarbon transport parameters. CFC-12 appears to be the most conservative of the CFCs measured. Sorption at this site is low ( K d < 0.03), and degradation does not appear to be important. CFC- 113 is retarded both with respect to CFC-12 and with respect to 3 H ( K d = 0.09−0.14). CFC-11 appears to be degraded both in the highly organic unsaturated zone and below 3.5 m depth in the aquifer, where dissolved oxygen concentrations decrease to below 0.5 mg L −1 . The half-life for CFC-11 degradation below 3.5 m depth is less than 2 years. While apparent CFC-12 ages match hydraulic ages to within 20% (up to 30 years), apparent CFC-11 and CFC-113 ages significantly overestimate hydraulic ages at our field site.

Water Resources Research↗
USGS2010

Inferring shallow groundwater flow in saprolite and fractured rock using environmental tracers

The Ridge and Valley Province of eastern Tennessee is characterized by (1) substantial topographic relief, (2) folded and highly fractured rocks of various lithologies that have low primary permeability and porosity, and (3) a shallow residuum of medium permeability and high total porosity. Conceptual models of shallow groundwater flow and solute transport in this system have been developed but are difficult to evaluate using physical characterization or short‐term tracer methods due to extreme spatial variability in hydraulic properties. In this paper we describe how chlorofluorocarbon 12, 3 H, and 3 He were used to infer groundwater flow and solute transport in saprolite and fractured rock near Oak Ridge, Tennessee. In the shallow residuum, fracture spacings are <0.05 m, suggesting that concentrations of these tracers in fractures and in the matrix have time to diffusionally equilibrate. The relatively smooth nature of tracer concentrations with depth in the residuum is consistent with this model and quantitatively suggests recharge fluxes of 0.2 to 0.4 m yr −1 . In contrast, groundwater flow within the unweathered rock appears to be controlled by fractures with spacings of the order of 2 to 5 m, and diffusional equilibration of fractures and matrix has not occurred. For this reason, vertical fluid fluxes in the unweathered rock cannot be estimated from the tracer data.

Water Resources Research↗
USGS2005

Ground water age and nitrate distribution within a glacial aquifer beneath a thick unsaturated zone

The impact on ground water quality from increasing fertilizer application rates over the past 40 years is evaluated within a glacial aquifer system beneath a thick unsaturated zone. Ground water ages within the aquifer could not be accurately determined from the measured distribution of 3 H and as a result, chlorofluorocarbon (CFC) and 3 H/ 3 He dating techniques were applied. Beneath a 25 m thick unsaturated zone, ground water ages based on CFC‐11 concentrations were greater than 3 H/ 3 He ground water ages by 6 to 10 years, due to the time lag associated with the diffusion of CFCs through the unsaturated zone. Using the corrected CFC‐11 and 3 H/ 3 He ground water ages and the estimated travel time of 3H within the unsaturated zone, the approximate position of ground water recharged since the mid‐1960s was determined. Nitrate concentrations within post mid‐1960s recharge were generally elevated and near or above the drinking water limit of 10 mg‐N/L. In comparison, pre mid‐1960s recharge had nitrate concentrations <2.5 mg‐N/L. The elevated NO 3 − concentrations in post mid‐1960s recharge are attributed mainly to increasing fertilizer application rates between 1970 and the mid‐ to late 1980s. Anaerobic conditions suitable for denitrifkation are present within pre mid‐1960s recharge indicating that removal of DO is a slow process taking tens of years. Over the next 10 to 20 years, nitrate concentrations at municipal well fields that are currently capturing aerobic ground water recharged near the mid‐1960s are expected to increase because of the higher fertilizer application rates beginning in the 1970s and 1980s.

Groundwater↗
USGS2002

Using groundwater levels to estimate recharge

Accurate estimation of groundwater recharge is extremely important for proper management of groundwater systems. Many different approaches exist for estimating recharge. This paper presents a review of methods that are based on groundwater-level data. The water-table fluctuation method may be the most widely used technique for estimating recharge; it requires knowledge of specific yield and changes in water levels over time. Advantages of this approach include its simplicity and an insensitivity to the mechanism by which water moves through the unsaturated zone. Uncertainty in estimates generated by this method relate to the limited accuracy with which specific yield can be determined and to the extent to which assumptions inherent in the method are valid. Other methods that use water levels (mostly based on the Darcy equation) are also described. The theory underlying the methods is explained. Examples from the literature are used to illustrate applications of the different methods.

Hydrogeology Journal↗
USGS1998

Evidence of deep circulation in two perennially ice-covered Antarctic lakes

The perennial ice covers found on many of the lakes in the McMurdo Dry Valley region of the Antarctic have been postulated to severely limit mixing and convective turnover of these unique lakes. In this work, we utilize chlorofluorocarbon (CFC) concentration profiles from Lakes Hoare and Fryxell in the McMurdo Dry Valley to determine the extent of deep vertical mixing occurring over the last 50 years. Near the ice-water interface, CFC concentrations in both lakes were well above saturation, in accordance with atmospheric gas supersaturations resulting from freezing under the perennial ice covers. Evidence of mixing throughout the water column at Lake Hoare was confirmed by the presence of CFCs throughout the water column and suggests vertical mixing times of 20-30 years. In Lake Fryxell, CFC-11, CFC-12, and CFC-113 were found in the upper water column; however, degradation of CFC-11 and CFC-12 in the anoxic bottom waters appears to be occurring with CFC-113 only present in these bottom waters. The presence of CFC-113 in the bottom waters, in conjunction with previous work detecting tritium in these waters, strongly argues for the presence of convective mixing in Lake Fryxell. The evidence for deep mixing in these lakes may be an important, yet overlooked, phenomenon in the limnology of perennially ice-covered lakes.

Limnology and Oceanography↗