2| 2-o 7 Observations and ^ instruments • Location of observation wells The number of observation wells necessary to study the ground-water regime, and their areal distribution, is dependent on the special features of the area to be investigated and the type of study planned. For example, if a thorough study of the natural occurrences and interrelationships of ground and surface water is intended for a basin (having an area of 100 km2 or less) the annual and seasonal ground-water balances as well as total water balances must be determined. Basins of this type may be considered as complex water-balance laboratories under natural conditions. Observations made in such basins allow trie determination and study of various natural processes that govern the quality and distribution of waters of various types. For a basin having an area of up to 1,000 km2 only the principal elements of the water balance are usually studied. These are precipitation, run-off, maximum snow storage and, in some cases, the ground-water regime and other elements. A general discussion of criteria for designing a network of observation wells is given later in this chapter in Section 7.1.2. Network planning was the subject of symposia at Quebec, Canada (WMO-IASH, 1965) and Berne, Switzerland (IASH, 1968). The special features of investigations and distribution of observation-well networks for the following types of ground-water regimes or aquifer systems will be considered: 1. Platform areas: (a) homogeneous areas; (b) heterogeneous areas; (c) complex areas. 2. Mountain-folded areas: (a) areas with strip-like occurrences of permeable rocks; (b) folded areas. 3. Permafrost areas. 4. Submontane terrains and valieys in arid regions: (a) pebbled (coarsest) part of the alluvial fan; (b) the middle part of the alluvial fan; (c) the subzone of unconfined ground-water discharge; (d) subzone of resubmergence and evaporation of ground • water; (e) subzone of alluvial ground water. 5. Alluvial and outwash plains (zone of excess moisture). 6. Karst areas. This classification of ground-water regimes is arbitrary and by no means comprehensive; however, it allows the development of guides for the planning and programming of observation well networks. % uyH,-^ Raf ..-;.•.••: Centre .-^u.r Ripply 7.1 page I Ground- water studies 7.1.1 Environments affecting the design of well networks 7.1.1.1 Platform areas The principal feature of the geological structure in platform areas is the horizontal attitude of sedimentary rocks which overlie the basement rocks and the general absence of tectonic disturbances. Ground water in the sedimentary rocks occurs commonly in per­ meable sands drained by streams and recharged primarily by infiltration of precipita­ tion. Homogeneous conditions. Under homogeneous conditions the sedimentary rocks of the area constitute commonly a single unconfined, or water-table, aquifer. Temporarily perched ground water may occur on clay lenses in the zone of aeration (Fig. 7.1.1.1a, 1.1). Streams drain the water-table aquifers completely or partially depending on the depth of their downcutting. The aquifers are recharged mainly by local precipitation and the topographic divide of the basin generally coincides with the ground-water divide. The base flow, K&, from a basin of this type may be determined by separating a streamflow hydrograph obtained at the basin outlet. If the under-channel run-off at the outlet is less than 5 per cent of the base flow, it need not be considered in the stream basin balance. By analysis of water-table fluctuations the value of ground-water recharge from above, WAt, can be determined, and knowing the change in ground-water storage, nAH, the value of ground-water flow, Qu, from the drainage basin area may be deter­ mined from the following equation: Y, - Q„ -= ^Vw.av. M ~ 2(A/,//)w-av- 7I(I) r r where summations represent the products of the weighted average values of recharge, fVv/.^v./lt, and the weighted averages values, (uAH)w.av., of the change in ground-water storage. The unconfined ground-water balance for the basin is composed of the weighted average (in respect to area) values of infiltration of precipitation, Iw.av. Ax; values of evaporation from the water table, U •*.•*•<. A V; and values of the differences between the inflow Q\ and outflow Qi of unconfined ground water \{Q\ — Qi)lF\«.v,.At. Values for these elements are calculated from data obtained from ground-water regime observa­ tions made on calculation (balance) plots. Thus the unconfined ground-water balance is determined by use of the equation: Ql Q2 y (UAH)*.-.**. - y ( ~ \ At -'- y /»,,>-. AX - y uK.^. AV IMD 4- ^\ F Av.av. — ^ The total water balance for the basin is determined from the formula: (nAHU- -'• Wx -• Wt =- X - Z + Ki •- Ka.ov - <3n 7.1(3) where Y2.W =-• Y» - Qu, is the overland flow from the basin. Heterogeneous conditions. Under heterogeneous conditions there are two or more discrete aquifers in the basin in addition to any temporarily perched water. The base flow is composed of ground-water flow from the first and second aquifers (Fig. 7.1.1.1a, 1.2). As the ground water in the second aquifer results from leakage from the first aquifer the total base flow may be estimated either by stream hydrograph separation (as in the case of homogeneous conditions) or by use of Equation 7.1(1). The unconfined ground-water balance differs from that for homogeneous conditions because of the vertical water exchange between aquifers as indicated by the differences in hydraulic head in the first and second aquifers. The amount of this exchange between aquifers is dependent on the hydraulic conductivity of the intervening semi-pervious bed 7.1 page 2 Observations and instruments FIG. 7.1.1.1a. Typical diagrams of structure in representative basins PLATFORM AREAS, i.l. Homogeneous conditions Geological and hydrogeological conditions Stream hydrograph separation ' , • Impermeable bed !hnTT*im '•//• • 1111 nfrjirt i i n i nil in^rhiri r r'mrH^'i A jingle main aquifer of temporary perched water. All ground-water flow drained bv the stream. n- Pattern of base flow Yt, (mm/year) I t Pattern of the unconfined ground-water balance for a basin with an area A (mmjyear) Qi- -Ga< l a /wav dr (fiAH)w.av. — ^( f-- ) <*' "I" ^ ' - ^ ^w»v./) I t I Pattern of the total water balance for a basin with an area A (mm/year) (liAH)w. + Wx + W2 - X - Z -, Kx ~ K2,ov. - Qn where: ^.ov. — Y2 — Qu Type c(base flow regime: Primarily derived from bank storage. 7.1 page 3 Ground-water studies FIG. 7.1.1.1a. Typical diagrams of structure in representative basins (cont.) i. PLATFORM AREAS. 1.2. Heterogeneous conditions Geological and hydrogeological conditions Stream hydrograph separation . Impermeable bed 7 7777777 777777777 Two aquifer! apart from temporary perched water. Ground-water flow from the lower aquifer it equal to trie amount of leakage from the upper aquifei. Pattern of base flow Y>, (mm/year) Yb = <2ii ->- Qii -= ^ W»- At - 2jnAH)v.av. i t Pattern of the unconfined ground-water balance for a basin with an area A (mm)year) ivJr Pattern of the total water balance for a basin with an area A (mm/year) (tiAH)„. + Wi + W2 = X - Z - Ki - K2,ov. - <?i'i - Oil where: ^.ov. = Yz — Qu — Q\\ Type of base flow regime: Primarily from bank storage. 7.1 page 4 Observations and instruments FIG. 7.1.1.1a. Typical diagrams of structure in representative basins (cont.) i. PLATFORM AREAS. i.3. Complex conditions Geological and hydrogeological conditions Stream hydrograph separation Pattern of base flow Y\> (mm/year) Yb = Q&so. - On + Qu — Ziter. — Zn.pi. At X * 0; AH * 0, Qu - Qu 4- Qu + AVi- -ZI-I- - Y2.ov. - (^tf)av t-i- Gasc. — Yb — Qu (Proved by hydrodynamic calculation.) Pattern of the unconfined ground-water balance for a basin with an area A (mm/year) The same as before, for unconfined ground water, but for separate regions with conditions which are similar to diagrams 1.1 and 1.2. The ground-water balance is made individually for separate regions. Pattern of the total water balance for a basin with an area A (mm/year) The same as before, for the part of basin that is to the right of section I-I'. For all the basin: (^tf)av. -.-Wi + Wi^-X-Z-Ki- K2,ov. - Qi - Qu = = AW I Zi-\. + Y2,ov.l-V + (^tf)av.I-I' H C?afc Type of base flow regime: Cascading springs and seeps and bank storage, and artesian discharge. 7.1 page 5 Ground-water studies FIG. 7.1.1.1a. Typical diagrams of structure in representative basins, (cont) II. MOUNTAIN-FOLDED AREAS, ii. 1. Areas with a strip-like occurrence of permeable rocks Geological and hydrogeological conditions Stream hydrograph separation Ji1,,'^ • 1/ Pattern of base flow K& (mm/year) t Pattern of the unconfined ground-water balance for a basin with an area A (mm/year) The ground-water balance is made for individual tributary basin-blocks of hydrogeological structure. V(//JW)w.av W Ai ( K-^L/' - 2 — Pattern of the total water balance for a basin with an area A (mm/year) (fiAH)av. + Wi = X-Z~Ki-Yi Wz = 0 Y-i =-- K2,ov. + Yb (from separation of streamfiow) Type of base flow regime: Cascading springs and seeps and bank storage. 7. t page 6 Observations and instruments FIG. 7.1.1.la. Typical diagrams of structure in representative basins (cont.) II. MouNTAiN-FOLDiiD ARKAS. ii.2. Folded areas Geological and hydrogeological conditions Stream hydrograph separation PLAN SECTION Pattern of base flow Yt, (mm/year) Yb - y WVav.^ --• X- Z ~ K\- K2.„v.