PINSTECH-171 INNOVATIVE TECHNIQUE FOR ASSESSMENT OF GROUNDWATER QUALITY Niaz Ahmad Manzoor Ahmad M. Ishaq Sajjad RADIATION AND ISOTOPE APPLICATION DIVISION PAKISTAN INSTITUTE OF NUCLEAR SCIENCE AND TECHNOLOGY P.O. NILORE, ISLAMABAD July 2001 CONTENTS ABSTRACT INTRODUCTION GEO-HYDROLOGY RESULTS AND DISCUSSIONS 3.1 Hydro-chemical Data 3.2 Classification of Chemical Analyses 3.2.1 Classical Trilinear Diagrams 3.2.2 Modern Multi-Rectangular Diagrams 3.3 Hydro-Chemical Spatial Variations 3.4 Sodium-Calcium Relationship 3.5 Hydro-geochemical Processes CONCLUSIONS ACKNOWLEDGEMENTS REFERENCES ABSTRACT Groundwater quality of a part of Chaj Doab has been assessed with innovative techniques which are not reported in literature. The concept of triangular coordinates is modified by multi-rectangular ones for the classification of major cations and anions analysed in the ground water. A MuJti-Rectangular Diagram (MRD) has been developed with the combination of rectangular coordinates by virtue of which milli-equivalent per liter percentages (meq/l %) of major cations and anions could be classified into different categories more efficiently as compared to classical trilinear diagrams. Both Piper diagram and MRD are used for the assessment of 259 data sets analysed from ground water of Chaj Doab area, Pakistan. The differentiated ground water types with MRD in the study area are calcium bicarbonate, magnesium bicarbonate, sodium bicarbonate and sodium sulfate. Sodium bicarbonate type emerges as the most abundant type of ground water in the study area. A map showing spatial variation of groundwater quality has been constructed with the help of MRD. This map shows that, in the vicinity of rivers Chenab and Jhelum, calcium bicarbonate type of waters occur while the central area is mainly covered by sodium bicarbonate dominant waters. Groundwaters near the upper Jhelum canal are dominant in sodium sulfate . An important relation between calcium and sodium is proposed which explains the movement history of groundwater in the aquifer. Hydrogeochemical processes have been evaluated with new methods. Ion exchange between calcium and sodium, precipitation of calcium bicarbonate and dissolution of rock forming minerals are the major delineated hydrogeochemical processes. INNOVATIVE TECHNIQUE FOR ASSESSMENT OF GROUNDWATER QUALITY 1. INTRODUCTION Groundwater can be considered a potential resource where it is available in exploitable quantity and quality. Storage and transmission of the reservoir rocks are the most deciding parameters about the assessment worth of the water resource. Especially, rocks deposited under alluvial environments, can be considered good candidates for the storage of water if sufficient recharge is available in the area from surface water bodies (rivers, lakes, canals) and precipitation (rain, snow). Quality of ground water goes on changing after percolation in the ground due to its high reactivity. Different rocks which meet in the way of flowing water, impart different chemical composition to the groundwater even recharged from the same surface source. Major chemical ions which it carries in the dissolved form are Ca, Mg, Na, K, HCÛ3, CO3, S04, CI, and N03. The quality of ground water is rated depending upon the purpose of its use in industry, domestic, livestock, agriculture etc. During the resource evaluation stage, delineation of spatial variation of quality is an imperative step. Graphical models such as trilinear diagrams (Hill, 1940; Piper, 1944; Durov, 1948; Burdon and Mazloum, 1958 and Lloyd, 1965) are mostly used for the classification of different groundwater qualities. Trilinear diagrams make use of triangular coordinates and despite their benefits, pose several difficulties in the final interpretations. A new graphical model developed by Ahmad (1998) is going to be introduced here for the classification of ground water compositions. In the new apprôach, rectangular coordinates instead of triangular ones have been used. This method is called Multi- Rectangular Diagrams (MRD) because of the rectangular coordinates. The study area has dimensions 80 km x 40 km and is a part of Chaj Doab. It is bound by rivers Jhelum and Chenab, and canals Upper Jhelum and Lower Jhelum. Agriculture is the major economic activity in the area. Water for irrigation is supplied by aforementioned canals and local tube wells. 1 The main purpose of this report is to introduce some innovative graphical models for the classification, spatial distribution and recognition of hydro-geochemical processes for grounwaters analysed in the upper middle part of Chaj Doab Area. 2. GEO-HYDROLOGY f The geologic formations in the study area mainly consist of sediments which were produced by the rapid erosion of rising Himalayas. These sediments were transported by present and ancestral tributaries of Indus River. The sediments were I deposited in a subsiding trough consisting of igneous and metamorphic rocks. Because of the constantly shifting courses of the depositing streams, the alluvium is of heterogeneous nature showing little continuity in lateral and vertical directions. The alluvium consists of unconsolidated sand and silt, with minor amounts of clay and gravel. Main constituent minerals are quartz, muscovite, biotite and chlorite, in association with a small percentage of heavy minerals, calcium carbonate nodules (locally called Kankars), a deposit of secondary origin is also associated with fine strata (Kidwai, 1963, Tipton, 1960 and Wadia, 1957). Thickness of total sediment pile is not known clearly as only alluvium has occurred in the test bore holes drilled to a depth of 500 meters in the area so far. The clay lenses are wide spread in the area. The presence of clay is very important from storage and transmission of water point of view as it is expected to impart confining pressures thereby producing artesian conditions at the time of aquifer exploitation. On the other hand, the possibility of perched aquifer conditions can not be ruled out to exist sometimes. 3. RESULTS AND DISCUSSIONS 3.1 HYDRO-CHEMICAL DATA The chemical data of groundwater wells used in this report has been obtained from Salinity Monitoring Organization (SMO), Water and Power Development Authority 2 (WAPDA), Mughalpura, Lahore. Data has been obtained on 257 wells and two rivers Jheium and Chenab. The river data was analysed by RIHG/RIAD of PINSTECH. The data sets consist of mean values of 10 year observations taken from 1978-88. Although the data is old but the purpose here is not strictly the evaluation of groundwater quality of a specific area. Here the underlying objective is the modification of trilinear graphical models into Multi-Rectangular graphical model. Data on major chemical ions such as Ca, Mg, Na, HC03, C03, S04, CI, K and total dissolved solids (TDS) is obtained from WAPDA. The quality of data has been ascertained by using meq/l concentrations of cations and anions in the ionic charge balance equation as V cations - V anions . reaction error(%) = ^ XI00 > cations + > anions The samples with reaction error less than 10 % have been accepted for interpretation. 3.2. CLASSIFICATION OF CHEMICAL ANALYSES The quality of ground water depends upon a number of dissolved chemical constituents and, generally, it is assessed by determining dominant cation and dominant anion associations. A quick overview of all the ground water analyses from any area of study can be comprehensively summarized as a presentation of all the significant chemical ions on a single diagram. Trilinear diagrams are being used for the underlying purpose. Here the same objective has been obtained more effectively by replacing the trilinear diagrams with the newly developed MRD's. 3.2.1. CLASSICAL TRILINEAR DIAGRAMS Although, a number of trilinear diagrams are available in literature but here the most popular one Piper diagram (Piper, 1944) is discussed. In Piper scheme, concentrations of each cation and anion are taken in milliequivalent per liter (meq/l). Although in laboratory analysis, concentrations are usually measured in parts per million (ppm) but conversion to meq/l takes into account the equivalent weights and charges of 3 the individual ions. As the chemical reactions consume and produce these ions in the natural environment according to their equivalent weights, therefore, this conversion to meq/l is a good approximation and helps' in determining the underlying hydrochemical process. The conversion of concentration from ppm to meq/l is possible by the following relation; ppm meq/l = equivalent weight where formula weight equivalent weight = charge number on the ion Finally, percentage of simple meq/l concentrations of individual ions is evaluated from each respective group of cations and anions separately. As an example, the percentages of Ca and (HCO3 + C03) are calculated as under: Ca Ca (%) =" x 100 (Ca + Mg + K + Na) and hco3+cqs (HCO3 + C03 ) (%) = x 100 (HCO3 +CO3 +S04 +a+N05) respectively, where all the ionic concentrations on the right hand side are taken in meq/l. Again, the use of percentages rather than simple concentrations is meaningful because percentage variable becomes a composite variable and carries the hidden effect of other partners also and one can decide easily about the dominance or non- dominance of a particular ion with respect to others. The Piper diagram possesses the interrelationships of multi-sets of chemical data and is constructed by making use of equilateral triangles. In actual practice, a larger single triangle is divided into two smaller triangles