『Abstract
In situ silicate dissolution rates within the saturated Navajo
sandstone, at Black Mesa, Arizona were determined from elemental
fluxes in the aquifer. The mass transfer between groundwater and
mineral matrix along flow paths was calculated from inverse mass
balance modeling. The reaction time is bounded by 14C-based
travel time. BET surface areas were measured with N2
gas adsorption. Dissolution rates for K-feldspar and plagioclase
are 10-19 and 10-16 mol (feldspar) m-2
s-1, respectively, which are 〜105 times
slower than laboratory experiment-derived rates under similar
pH and temperature but at far from equilibrium conditions. The
rates obtained in this study are consistent with the slower field
rates found in numerous watershed and soil profile studies. However,
these rates are from saturated aquifers, overcoming some concerns
on estimated rates from unsaturated systems. The Navajo sandstone
is a quartz-sandstone with a relatively simple and well-studied
hydrogeology, groundwater geochemistry, and lithology, a large
number of groundwater analyses and 14C groundwater
ages, groundwater residence times up to 〜37 ky, groundwater pH
from 〜8 to 10, and temperature from 〜15 to 35℃.』
『要旨
アリゾナ州のブラック・メサの飽和したNavajo砂岩内の原位置珪酸塩溶解速度が、帯水層での元素フラックスから決定された。流路に沿う地下水と鉱物基質間の質量移動が逆質量バランスモデルから計算された。反応時間は14Cに基づく移動時間により決められた。BET表面積がN2ガス吸着で測られた。K長石と斜長石の溶解速度はそれぞれ10-19および10-16
モル（長石）/m2/秒で、同様のpHと温度であるが平衡から離れた条件下で室内実験から得られた速度より〜105倍遅い。本研究で得られた速度は、たくさんの流域と土壌断面の研究で知られた遅い野外速度と矛盾しない。しかし、これらの速度は飽和帯水層からのものであり、不飽和系から見積られた速度に対していくつかの大事な点で勝っている。Navajo砂岩は石英砂岩であり、比較的単純でありよく研究された水文地質・地下水地球化学・岩相、大量の地下水分析と14C地下水年代、〜37000年までの地下水滞留時間、約8〜10の地下水pH、および約15〜35℃の温度である。』
1. Introduction
2. Geology, hydrology, and geochemistry of the N aquifer
2.1. Hydrogeology
2.2. Groundwater geochemistry in the N aquifer
2.3. 14C ages of groundwater
3. Field sampling and experimental methods
3.1. Bacterial counts
3.2. Electron microscopy
3.3. Surface area analysis
4. Results
4.1. Bacterial count
4.2. Microscopic observations of feldspar dissolution and clay
precipitation
4.3. Surface area measurements
4.4. Inverse mass balance modeling
4.5. Calculated feldspar dissolution rates
5. Discussion
5.1. Estimation of surface area
5.2. Biologic activity
5.3. Hypotheses for the discrepancy between field and laboratory
rates
6. Conclusions and remarks
Acknowledgments
References
Appendix A. Inverse mass balance modeling
Appendix B. Error analysis
Appendix C. Comparison with other field rates
C1. Field rates in aquifers from previous studies
C1.1 Crystal Lake aquifer
C1.2 A flank aquifer of Poas（aの頭に´） Volcano,
Costa Rica
C1.3 Cape Cod aquifer
C1.4 An unconfined aquifer in Northern Wisconsin
C2. Methodologies of mass balance calculations