Ground Water in the Norwalk, Suffield, and Glastonbury Areas, Connecticut

There are two principal sets of joints, one of which is nearhr hori- zontal and the other nearly vertical. The vertical joijits, according to Ellis,- are from 3 to 7 feet apart where jointing is well developed^ In some sheeted zones 1 to 15 feet wide the joints are spaced at in- tervals of 3 inches to 2 feet, but in some places they are 100 feet 1 Sorao of the geologic names used in this report (Thomaston granite gneiss. Danhury urauodiorito yneiss, Glastonbury granite gneiss, Waterbury gneiss, and Hebron gneiss) are the pvorisional names ^iven to the rocks on the proiiminary geologic map of Con- necticut by ClregGiy and Robinson (Connecticut Geol. and Nat. Hist. Survey Bull. 7, 1907). These names aie herein used only for reference and may differ from those which will finally be adopted by the United States Geological Survey in it:: geologic folios. " Gregory, H. E., and Ellis, E. B.. Undergiound-water i-esources of Conaecti-eut ; U. S. Geol. Survey Water-Supply Paper 2.32, p. 0.5. 1900. 32 GROUND WATER IN NOR WALK AND OTHER AREAS, CONN. t'.part. Though the spacing increases with increasing depth it is on the average less than 10 feet to a depth of 100 feet. Ellis finds that the horizontal joints are on the average 1 foot apart for the first '20 feet, between 4 and T feet apart for the next 30 feet, and from 6 to 30 feet apart at depths of 50 to 100 feet. The intersecting hori- zontal and vertical joints form a very complicated system of connect- ing channels through which water may circulate. Water is supplied to the network of channels by percolation from the overlying mantle of soil, and it may be recovered by means of drilled wells. LIMESTONE. DISTRIBUTION. The Stockbridge dolomite underlies about 9 square miles of valley land in the town of Ridgefield, in the Norwalk area. LITHOLOGY AXD STRATIGRAPHY. The Stockbridge dolomite is a metamorphosed .dolomitic limestone, composed chielly of calcite and dolomite, and for the most part has a thoroughly crj^stalline texture. Some zones, however, have been but slightly metamorphosed and have still the texture of a typical limestone. Because of the solubility of the calcite the rock has slight resistance to erosion and constitutes valley areas. It is one of the few formations in Connecticut whose age is definitely known, for it has been traced into regions in Massachusetts where fossils have been found. OCCURRENCE OF GROUND WATER. Water is carried in the Stockbridge dolomite in the same way as in the schists, gneisses, and sandstones, namely, in joints. The joints, however, have been in large part widened by the solvent action of the water floAving through them, so that they are excellent channels of circulation and should yield abundant supplies of vrater. It is to be expected, however, that the waters derived from this forma- tion will be rather hard. Unlike most dolomitic marbles Stockbridge dolomite has a very low porosity and carries but little water in pores. ARTESIAN CONDITIONS. The word " artesian '' is derived from the name of the old French province of Artois, in which wells of this type first became widely known. Originally the term was applied only to wells from which Avater actually floAved, but now it is applied to Avells in Avhich the AA'ater rises by hydrostatic pressure aboA^'e the point at which it AKTKSIAX ("OXDITIOXS. 33 enters the hole. The tei'in is sometimos imi)i-oi)erlY used for any deep AVC'II of small diameter, ie<2;ar(llcss of Avhether the water is under pre^>iire or not. The (jiiestion A\hetlier an artesian well will tiovc or not de!iend> as iniich on the altitude of the mouth of the well as it does on the pressure of the water. The essential conditions for artesian AA'aters are the existence of a bed of jiorous or fractured rock through Avhich water may flow, hav- ing an elevateil outcrop where water may soak into it, with relatively impervions strata above and beloAv to prevent escape of water and loss of pressure, and a snpply of water to the outcrop sufficient to fill the resei'voir. In Connecticut these conditions may be fulfilled in two principal ways — where sandstones between shales or sandstones between trap sheets function as the pervious and impervious strata, or where a blanket of compact till forms the restraining layer over bedrock that is perviou.s by reason of a network of fissures. In general, the rocks FiGCKE 6. -Diagram showing conditions under which artesian waters may exist in the Triassic sedimentary rocks in Connecticut. contain so many faults and open joints that the water escapes and it-^ pressure is dissipated, so that flowing wells are few. Nearly all the wells are artesian, however, for the water in them rises con- siderably above the point of entrance. A few wells pass through beds of relatively impervious shale and draw water from porous sandstone, as shown in figure 6. The under- lying restraining member may be either a shale bed, as at A, or the dense crystalline mass on which the Triassic beds rest, as at B. In general the beds of the Triassic sedimentary rocks are not of sufficient lateral extent to form important reservoirs. In a few wells a sheet fif trap rock forms the upper restraining member, as illustrated at C" in figure G. The wells that draw water from the network of fissures are much more numerous than those drawing from the pores of the sandstones and conglomerates. In some of these rocks there are no connecting joints that might discharge water beloAV the level of the wells; in 1.54444°— 20 3 34 GROUiS^D WATER IN NOEWAI^K AlN^D OTHER AREAS, 00^1^, otliers -the joints are tight enough to offer material resistance to the escape of water. Otlier wells draw water from fissiu'ecl rock that is 0¥erlain by an impervious blanket of till that acts as a restraining member. SPRINGS. A spring, in the broadest sense of the word, is a more or less definite surface outlet for the ground water. Springs are formed where the surface of the gromid is so low that it reaches the water table. A well is in a sense an artificial spring, for it is made by artificially depressing the ground surface so that it reaches the water table. Tlie springs in the areas covered by this report may be grouped under three principal heads, as described below, SEEPAGE SPRINGS. One method of escape of water from tlie ground is by slow seepage in saturated areas on hillsides and along swamps and streams. This process may go on over a wide space if the formation is of uniform texture, or it may be concentrated in a small body of more porous material. To the latter class belong the so-called ^' boiliu-g springs,'' in which the water enters with sufficient force to keep the sand Iwt- tom in gentle motion. In a spring of either class the supply may be artificially concentrated by the excavation of a colleetiiig reservoir. Seepage springs are very likely to be found in small swales cut back into a slope. It seems probable that the flow" of w^ater is the primary cause of the excavation of the swales, but that the swales secondarily tend to concentrate the flov/. Areas of diffused seepage may develop into true springs by such a process. STSATUM SPRINGS. Stratum springs are those in which an outcropping or only slightly buried ledge or layer of impervious material interrupts the flow of ground water and forces it to the surface. Springs of this type may be made by a ledge of rock underlying saturated soil, by beds of sedimentary rock of different porosity, or by a body of till underlying stratified drift. FAULT AND JOINT SPRINGS. Faults and joints greatly facilitate the circulation of water through rocks, and where they reach the surface they may supjoly springs. Some faults carry a good deal of water under considerable pressure and are in a sense analogous to artesian wells. RELATIONS OE WELLS TO SPRINGS. Wells may be considered artificial springs. (See above.) Some springs that have been improved by ^excavation to a considerable depth are hard to distinguish from wells that have obtained water EEOOVliRY OF GROUXl> WATER. 35 !it moderate depths. In this repoi-t the criterion t^iken for classi- I'vin^' such spiinas i-s the original condition of the ground. If it ap[)ears to have been a wet or springy spot, the term "spring" is applied regardless of the depth of excavation. If the surface ^^as dry in the first jdace, the term " well '• is applied no matter how shallow the depth at which the crater table was found. RECOVERY OF GROUND WATER. DUG WELLS. COXSTRUCTIOX. Dug wells are constructed by digging holes in tlie ground deep enough to extend below the water table. The excavation is gen- erally made 8 or 10 feet in diameter, and in it is built a lining of dry or inorttired masonry or brickv>ork, concrete, vitrified tile, or j>lanking. As the well is walled up the space outside the lining is filled. The filling should be of some porous material, such as gravel, in order to facilitate percolation into the well, but many well diggers pay little attention to this matter. Most dug wells when completed are 3 to 5 feet in diameter, though some are much larger, and their depth may be as much as 50 feet or more. The average depth of the 707 dug wells tabulated in this report is 18.3 feet, and they contain on an average al>out 5 feet of water. LTFTIXG DEVICES. A number of different devices are in use for raising water from dug wells. All are types or modifications of a simple bucket for ])ailing out water, the displacement pump, the impeller pump, or the siphon.