# 1 Introduction

The purpose of this Groundwater Project book is to introduce basic concepts in groundwater science including terminology and equations that will provide readers with the foundational knowledge upon which they can:

- characterize hydrogeologic properties of earth materials;
- compute groundwater flow rates, fluxes and velocities;
- understand the development of equations that represent groundwater flow; and,
- interpret and visualize groundwater flow patterns.

This book explains how the size, shape and interconnectedness of voids in Earth’s subsurface materials provide the framework for the storage and transmission of water found in the saturated zone below Earth’s surface. To describe this transmission, in the mid 1800’s, Henry Darcy developed a formula to calculate the volumetric rate of water flowing through sand from easily made measurements of head, gradient and hydraulic conductivity.

This book also introduces hydraulic head and shows that hydraulic head gradients drive groundwater flow in a manner similar to the way temperature gradients drive heat flow. The ease of groundwater flow through the subsurface is controlled by the intrinsic permeability of the geologic material as well as by the density and viscosity of the groundwater. Together these properties of the geologic material and water define the hydraulic conductivity of the earth material. When hydraulic head gradients are combined with the groundwater flow equations and hydraulic conductivity, the volumetric flow rate and average linear velocity of the groundwater is defined.

Darcy’s Law and the law of mass conservation underpin the mathematics used to quantitatively describe groundwater systems. The use of these two principles to develop equations representing groundwater flow under steady and transient conditions is described herein.

Geologic materials and formations that contain and yield groundwater for useful purposes are referred to as aquifers. Unconfined, confined, and perched aquifer systems are explained in this book. Aquifer properties such as transmissivity, specific storage and storativity are defined and their utility is illustrated.

Groundwater flow systems are examined, and the methods used to illustrate groundwater flow directions and patterns are presented. Controls resulting from physical and hydraulic boundaries are described as well as flow in isotropic, anisotropic, homogeneous, and heterogeneous settings.

The introductory material presented in this book provides a beginning for a reader’s journey into the hydrogeological world. Other Groundwater Project books address many of the subjects presented here in additional detail and introduce new concepts and materials.