Yerevan State University, Geological Department, Alex Manoogian 1, Republic of Armenia

Abstract. The Aparan confined aquifer feeds the aqueduct supplying Yerevan's drinking water. Hydrogeologically, the terrain consists of an unconfined or water-table aquifer composed of recent alluvial-proluvial sediments, and a confined aquifer composed of Quaternary lacustrine-alluvial sediments. An assessment of the natural and bacteriological vulnerability of these aquifers was made, and a 3D hydrostratigraphic model was constructed. We have conducted the analyses according to depths to water level from land surface and the thickness of poorly permeable soils in the aeration zone. According to this method, the following types of soils have been distinguished in the cross-section of the aeration zone: (1) K = 0.1-0.01 m/day (loamy sand or fractured matrix rocks); (2) K = 0.01-0.001 m/day (loams or relatively confining matrix rocks); and (3) K < 0.001 m/day (clay or confining matrix rocks), where K is permeability (conductance). The quantitative assessment of the unconfined aquifer's vulnerability is being carried out when there is risk of bacteriological infection. This method is based on the downward time-of-travel to the groundwater, during which percolated bacteria or polluted water may reach the water table. The quantitative assessment of the confined aquifer's vulnerability that is nearest to the land surface is carried out on the basis of two parameters: total thickness of the overlaid soils that have slow infiltration rates (m0) and the interrelation between level of above water-table (H1) and established level of water of aquifer under study (H2). A 3-D conceptual hydrostratigraphic model of the study area was constructed. As a result of evaluation, the unconfined aquifer was assessed as highly vulnerable, but the confined aquifer as protected by natural conditions.

Keywords: Aquifer, vulnerability, aeration zone, permeability, bacterial pollution 1. Introduction

The aquifer is located in an area of tremendous economic activity, affecting both the surface and subsurface. These activities, along with other natural exogenous processes, have a negative impact on hydrogeological conditions,

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mainly on the aquifers, which results in contamination and depletion of storage volumes. Currently, the contamination hazard of fresh water aquifers exceeds the danger of the storage depletion many times over. Hence, conservation of the ground water as a sustainable resource for drinking water supply is of paramount importance. Ground water is the only source for industrial and drinking water supply in the Republic of Armenia. The ground-water recharge areas are in the mountain ranges that naturally discharge water from the lower slopes and in the intermontane basins. Unconfined groundwater conditions occur on slopes. Aquifers in intermontane basins have both confined and unconfined nature.

Agriculture, including crop production and cattle-breeding, is developed in the mountain areas, 1,600 m above sea level and higher. Industrial manufacturing and agriculture, including horticulture, vegetable growing and growing of other industrial crops are developed at intermontane basins and foothills. All these activities are potential sources of groundwater contamination (Bochever, 1979). Ground water contamination is understood as changes in water quality under the impact of human activities compared with the quality of water in natural conditions, the "background level", that leads partially or entirely to the water being no use for human activities.

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