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

yranite wliicli has been injected at different times and subjected to intense nietannn-pliisni while yet deeply buried within tlie earth. On this hypothesis the more granitoid phases are most like the original rock, and the schistose phases are most metamorphosed. The horublendic and granitic beds were intruded before or during the chief metamorpliic movement, and owe their position and alinement to the forces that produced tlie main foliation. Veins of quartz and pegmatite were intruded after most of the metamorphism had taken place, and certain intrusions indicate even a later stage of igneous activity. That it is of igneous origin, however, is not certain, for the evi- dence of its original character has been largely destroyed by meta- morphism. By far the greater part of the town is underlain by the Thomas- ton granite gneiss. It is a true granite in that the dominant min- erals are quartz, feldspar, and black mica, but like the other rocks of the region a gneissic texture has been imposed on it by meta- morphism, as is shown by the bands rich in dark mica which alter- nate with bands rich in light quartz and feldspar. Tn some places phenocrysts of feldspar, which give the rock a porphyritic character, are developed. The rock is light gray in general, but some parts have a pinkish tinge. - ^ Rice, W. N., and Gregory, II.. E., Manual of the geology of Connecticut: Connecticut Gcol. and Nat. Hist. Survey Bull. G, p. CJ, 1906. 78 GROUND WATER IN" jS^OSWALK AI->7D OTHER AREAS, CONlSr. The capacities of these four rock types for carrying water are about the same and may be discussed tog-ether. Because of their low ]3orosity there is no appreciable amount of interstitial water. The schists are probably a little more porous than the gneisses be- cause of the presence of thin flat openings between the flakes ol mica, but even this is inconsequential. The crustal disturbances to which this region has been repeatedly subjected have produced many joints: and fissures vfhich constitute an intricately interconnecting network of narrow but extensive channels. Other joints are due to shrinkage during the initial consolidation of the rock. When rain fails on the ground a portion of it is absorbed and slowly percolates downiward. Most of this vf ater, when it reaches the bedrock surface, will move approximately horizontally^ but some will find its way into the joint system. In the upper zones of the bedrock the fissures are far more abundant than at greater depths, where they tend to be closed by the weight of the overlying rock. The water in the joint system may be recovered hj drilled wells. It is highly probable that at any specific point one or more water-bearing fissures will be intersected before drilling beyond 250 to 300 feet. If lione is cut it is better to try again in a new place than to drill deeper,, for it ia far less probable that a fissure will be cut betvfeen 300 anel 600 feet in the old place. The depth of the drilled wells in New Canaan for which data were obtained averages 173 feet and ranges from 86 to 300 feet. The yield of 14 wells averages 23 gallons a minute and ranges from 2 to 70 gallons. A few dug wells blasted down into rock also draw water from fissures, but they are not satisf actor;/ in general. The fissures very near the surface of bedrock are very apt to fail in drought, but the blasted cavity is of some value in that it acts as a reservoir and stores som.e water. Till. — Everywhere in New Canaan, except in parts of the valley floors and where ledges of rock outcrop, the bedrock is covered b}" a mantle of till. The depth to Ixedrock in 13 of the drilled wells tabulated below averages 37 feet and ranges from 7 to 79 feet. These fi^gures give some idea of the thickness of the till mantle. The till or boulder clay, or " hardpan " as it is locally called, is of glacial origin. The continental glacier which overrode this region from north to south plowed up and scraped away the soft rock and resid- ual soil and even removed some of the deeper unweathered rock. Projecting ledges and knobs of rock were torn away. The rock surface was smoothed, grooved, and polished by rock fragments em- bedded in the ice, and they in turn crushed, beveled, and polished one another. The resulting material, a heterogeneous mixture of pieces of rocks of many kinds and of all sizes from the very small NEW CANAAN. 71) particles of rock flour up to bouKU'is weighiu^- se\eial tons, was plastered over the glaciated surface. Deposition was particularly ooucentrated iu depressions and against the slopes of Icnlges and sl.arp ridges. The intimate niixmg of particles that vary greatly in size makes Iho product low in porosity. There are no openings within the peb- blo^^ and boulders tliat could hold water, and the spaces between them are filled with smaller particles. Moreovei-. the presence of fhie material means that the spaces that do exist are very small. JJowever, there is a very appreciable pore space, and a considerable amount of rain water is absorbed and held. On account of the [small size of the pores the water is transmitted very slowly, but wells dug in till will obtain supplies that are as a rule fairly reliable. Sixty-five such wells were visited in September and October, lOK'), in New Canaan. The measurements made of the v:ells are tabulated on pages sL-»2. The following table sunuiiarizes the data : t<i!ininar[/ of iccUs lUcj in iill in .Ye/.- ('minai:. Total depth. Depth. ;o "(Vater, Maxiniuu: Minimum Averas^o.. Feet. Feet. 29.5 5.2 U.4 Depth of vvater in well. F:Ct. 23.8 0.5 h.'2 The reliability of 51 of these wells was ascertained ; 33 were said to be nonf ailing and 18 to fail. The springs in the table on page 83, with the exception of No. 56, are in till. Stratified <?r^/^.-^Stratified drift, which may replace till as the mantle rock, is found in New Canaan only in parts of the valley floors. The map (PL III) shows the areas of stratified drift along Rippowam, Fivemile, and Silvermine rivers. This type of de- po.sit has been formed by the action of running w-ater as is shown by the well-washed, vsorted, and stratified character of the material. The source of the material is in large part till, but a little of it has undoubtedly been derived directly from the bedrock. The agent has been for the most part the streams that now occupy the valleys. In other parts of the State large volumes of melt water, derived from the receding glacier, built up extensive deposits of stratified drift. How^ever, the questions of the precise source of the material and of the particular transporting agency are not essential to a consideration of the deposits as a source of ground water. The impoi-tant facts are its character and distribution. It is char- 80 GEOUXD WATER IX XOEWALK AKD OTHEE AEEAS, CONN". acterized by sorting into distinct beds within which the particles are of relatively nniform size. Different beds and even adjacent beds are of very dissimilar coarseness. The absence of small particles from the interstices of the larger ones makes a very porous deposit capable of holding and rapidly transmitting a large amount of water, so stratified drift is an excellent source of ground water. Where the slopes are relatively steep the movement of water through stratified drift may be so rapid that wells in it may fail in dry seasons. No wells in stratified drift were ^dsited in New Canaan. Spring 56 (see map, PL II, and table, p. 83) is in stratified drift. QUALITY or GROUND WATER, The subjoined table gives the results of two analyses and three assays of samples of ground water collected in the town of New Canaan. The waters are low in mineral content except No. 68, which is moderately mineralized. Nos. 11, 18, and 62 are very soft; No. 35 is soft; and No. 68 is hard in comparison with other waters of this area, though it is not hard as rated by general standards. In so far as mineral content may detennine it, these waters are good for domestic use. All are good for boiler use except No. 68, which is rather high in scale-forming ingredients and is therefore rated as fair. The waters are calcium-carbSnate in type except Nos. 11 and 62, which are sodium-carbonate waters. Chemical composition and classification of ground wuters in New Canaan fi [Parts per million; collected Dec. 9, 1916; analyzed by Alfred A. Chambers and C. H. Kidwell. Numbers of analyses and assays correspond to those used on PI. II.] Silica (SiO;;) Iron (Fe) , Calcium (Ca) Magrnesiura (Mg) Sodium and potassium (Na+K)<i . Carbonate radicle (CO3) Bicarbonate radicle (HCO3) Sulphate radicle (SO4) Chloride radicle (CI) Nitrate radicle (NO3) Total dissolved solids at 180° C ... Total hardness as CaCOs Scale-forming constituents <i Foaming constituents d Chemical character Probability of corrosion f . Quality for boiler use Quality for domestic use . AnalTses. 6 23 .34 6.7 2.9 11 .0 24 16 4.6 12 S5 rf29 48 30 35 29 .18 14 4.7 15 .0 68 13 12 1.9 122 d54 78 40 Na-C03 Ca-COa (?) N Good. Good. Good. Good. Assavs. c 62 Trace. 7. .0 31 9.0 4.8 d73 28 55 20 Ca-COs (?)^ Good. Good. 12 .0 38 7.0 5.6 d77 22 45 30 Na-COs N Good. Good. 0.78 10 10 119 12 6.1 dl70 115 140 30 Ca-COs (?) Fair. Good. « For location and other descriptive information see pp. 81-83. ti For methods used in analyses and aecuraey of results see pp. 52-60. e .Approximations; for methods used in assays and reliability of results see pages 52-60. <i Computed. « Based on computed quantity; (?)=corrosioa uncertain; N = noneorrosive. NEW CANAAN. 81 ruBT-ic A\ A ir.i; surrLY.