『Abstract
Both chemical weathering rates and fluid flow are difficult to
measure in natural systems. However, these parameters are critical
for understanding the hydrochemical evolution of aquifers, predicting
the fate and transport of contaminants, and for water resources/water
quality considerations. 87Sr/86Sr and (234U/238U)
activity ratios are sensitive indicators of water-rock interaction,
and thus provide a means of quantifying both flow and reactivity.
The 87Sr/86Sr values in ground waters are
controlled by the ratio of the dissolution rate to the flow rate.
Similarly, the (234U/238U) ratio of natural
ground waters is a balance between the flow rate and the dissolution
of solids, and α-recoil loss of 234U from the solids.
By coupling these two isotope systems it is possible to constrain
both the long-term (ca. 100's to 1000's of years) flow rate and
bulk dissolution rate along the flow path. Previous estimates
of the ratio of the dissolution rate to the infiltration flux
from Sr isotopes (87Sr/86Sr) are combined
with a model for (234U/238U) to constrain
the infiltration flux and dissolution rate for a 70-m deep vadose
zone core from Hanford, Washington. The coupled model for both
(234U/238U) ratios and the 87Sr/86Sr
data suggests an infiltration flux of 5±2 mm/yr, and bulk silicate
dissolution rates between 10-15.7 and 10-16.5
mol/m2/s. The process of α-recoil enrichment, while
primarily responsible for the observed variation in (234U/238U)
of natural systems, is difficult to quantify. However, the rate
of this process in natural systems affects the interpretation
of most U-series data. Models for quantifying the α-recoil loss
fraction based on geometric predictions, surface area constraints,
and chemical methods are also presented. The agreement between
the chemical and theoretical methods, such as direct measurement
of (234U/238U) of the small grain size fraction
and geometric calculations for that size fraction, is quite good.
1. Introduction
2. Site description
3. Analytical procedures
3.1. Surface area measurements
3.2. Sequential leaches
3.3. Grain size fractions/mineralogy
3.4. Isotopic analyses
4. Results
4.1. (234U/238U) in pore waters and
leaches
4.2. (234U/238U) in bulk sediment and grain
size fractions
5. Discussion
5.1. Interpretive model of U isotopes in pore fluid
5.2. The α-recoil length of 234Th in natural minerals
5.3. Approaches to estimating the α-recoil loss fraction
5.3.1. Geometric considerations
5.3.2. Geometric estimates of the α-recoil loss fraction in
natural sediment
5.3.3. Measured depletion in fine-grained sediments
5.3.4. Measured depletion in mineral surfaces
5.3.5. Surface depletion and non-steady state α-recoil loss
5.3.6. Preferential leaching
5.4. Summary of α-recoil loss rate estimates for Hanford sediments
5.5. Estimates of the infiltration and weathering rate for Hanford
sediments
5.5.1. Previous estimates of infiltration and dissolution from
pore water 87Sr/86Sr
5.5.2. Coupled U-Sr model for dissolution/infiltration
6. Conclusions
Acknowledgments
References