Groundwater vulnerability mapping: A new mapping method as a useful decision making tool for oil and gas industry
Authors: Abdallahelhadj Mohamed Amine , Dhina Okba 
Published: February 2016

Non renewable groundwater resources in some dry areas are the unique source of water for all activities and sectors and therefore constitute strategic reserves for more than ever with the expected water shortage in the near future.

Contamination due to the infiltration of pollutants coming from oil and gas industries is one of the major risks that are jeopardizing these valuable resources.

Various systems of vulnerability evaluation have been developed so far. This paper discusses the widespread methods prior to introduce and discuss the new mapping method we propose for oil and gas upstream activities.

The rationale for the DKR overlays mapping method is linked to time of travel and the quantity of contaminants that move from the surface to the aquifer. The vulnerability is linked to the arrival of a contaminant at the water-table. This easy-to-implement method assesses vulnerability with limited consideration of the specific characteristics of the hydrogeological system or the behavior of contaminants. The three key parameters considered are:Depth to water-table (D);Hydraulic conductivity (K) of the upper geologic layers in the unsaturated zoneThe retention capacity (R) of the geologic formations of the unsaturated zoneThis simple yet useful method is developed to be used as a decision making tool when dealing with groundwater protection in oil and gas activities.

Introduction

Algeria is facing water scarcity problems so that water resources are well below the threshold set by the World Bank1. The annual supply of water per capita is less than 35 m3 and demand is by far exceeding supply2. Thus, it is crucial to preserve and protect all strategic Groundwater Reserves (G.R.) not only for meeting the current demand, but mainly, to preserve them for coming generations.

Since E&P activities in Algeria are mainly located in the Sahara where substantial G.R. spread out, aquifers vulnerability assessment is of high importance and a key step for an effective protection of these invaluable resources.

Thus, one of the main objectives of this paper is to develop an index - based approach in assessing the sensitivity of the aquifers to potential pollution and to illustrate the process leading to vulnerability maps that can be used as a decision - making tool.

The index - based approach developed along this paper mainly considers the intrinsic characteristics of the "Receptor" media (the aquifers) and the most likely contamination "Pathway" (the unsaturated zone).

Algerian Sahara Non Renewable Groundwater Resources.

The Non - Renewable G.R of interest belong to what is known as the North-Western Sahara Aquifer System (NWSAS) which covers an area of more than 1 million km² (1,800 km E-W, and 900 km N-S) and extends over Algeria, Tunisia and Libya3 (see figure 1). The NWSAS is essentially composed of continental deposits in which occurs two aquifer layers4:The Continental Intercalaire (CI) formation, which is the deepest, the thickest and has the greatest expanse,The Complex Terminal (CT) which is the upper part and is constituted of the Mio-Pliocene, Eocene, Senonian carbonate, and Turonian aquifers.