GROUND WATER IN NORTH-CENTRAL TENNESSEE
GROUND WATER IN NORTH-CENTRAL TENNESSEE
GROUND WATER IN NORTH-CENTRAL TENNESSEE
You also want an ePaper? Increase the reach of your titles
YUMPU automatically turns print PDFs into web optimized ePapers that Google loves.
OCCURRENCE OF <strong>GROUND</strong> <strong>WATER</strong> <strong>IN</strong> UMESTONE 71<br />
Solution openings in limestone are of two kinds (a) intergranular<br />
spaces produced by etching the faces of crystals or grains of rocks<br />
that have some primary porosity, by selective leaching of the more<br />
soluble constituents from such nonhomogeneous rocks as magnesian<br />
limestone and gypsiferous limestone, or by leaching the cement<br />
between the insoluble grains of a calcareous sandstone; (b) tubeHke<br />
channels formed in pure massive limestone by enlargement of joints<br />
or of primary openings along bedding planes. The tubular solution<br />
openings are the type commonly found in the limestones of north-<br />
central Tennessee.<br />
Calcium carbonate, which makes up practically all of a pure lime<br />
stone, is almost insoluble in pure water. However, it is appreciably<br />
soluble in rain water, which contains small quantities of carbon<br />
dioxide absorbed from the air. Also, it is especially soluble in soil<br />
water, which commonly contains an abundance of carbon dioxide<br />
produced by decomposition of the unstable organic acids leached<br />
from partly decomposed vegetation.42 Under favorable conditions<br />
these natural solvents may percolate considerable distances before<br />
they are neutralized. Rain water or soil water may enter joints or<br />
bedding planes and convert tight fractures into relatively open<br />
crevices * by etching the limestone walls, or it may enter primary pore<br />
spaces and enlarge them by etching the faces of crystals or grains.<br />
The rate at which the limestone is etched depends upon many factors,<br />
of which the principal are the chemical composition and crystallinity<br />
of the limestone, the permeability of the limestone, the rate of cir<br />
culation of the ground water, the amount and seasonal distribution of<br />
rainfall, the concentration of carbon dioxide and natural acids in the<br />
ground water, the thickness, composition, and texture of the soil,<br />
and the type and density of vegetal cover. For the etching to be<br />
continuous it is essential that the circulation be free, so that the<br />
saturated water will be continuously displaced by unsaturated water<br />
at the surface of the limestone.<br />
As is pointed out by Fuller,44 the sheet form of solution passage is<br />
the first stage in the enlargement of fractures or bedding planes in<br />
dense limestone. The small solution openings first formed eventually<br />
unite into an exceedingly narrow sheetlike opening which by differ<br />
ential solution may develop into a large irregular cavity. (See pi.<br />
5, A) Under favorable conditions solution may progress until the<br />
limestone is ramified by a network of caverns, some of which may<br />
grow to great size. The smaller channels commonly follow joints or<br />
bedding planes, but some of the larger channels and caverns do not<br />
« Murray, A. N., and Love, W. W., Action of organic acids on limestone: Am. Assoc. Petroleum Geolo<br />
gists Bull., vol. 13, pp. 1467-1475, 1929.<br />
« Spencer, A. C., U. 8. Qeol. Survey Press Bull. July 17,1922.<br />
« Fuller, M. L., Summary of the controlling factors of artesian flows: U. S. Qeol. Survey Bull. 319,<br />
p. 12, 1908.