**1. Introduction**

Groundwater (GW) belongs to all subsurface water, including saturated and unsaturated zones. More than 1.5 billion inhabitants around the globe depend on the groundwater for agriculture usage and industrialization consumption. However, pollutions were identified as one of the major challenges in hampering GW withdrawal (**Figure 1**) [1]. The exchange of the chemical and physical characteristics in water will affect the quality of groundwater resources, hence leading to the availability of humans in terms of quantity [2].

Groundwater is deposited between the pore spaces of rock/soils, cracks, joints, and fractures and various geological formations. The movement of groundwater in soils and rocks depends on the hydraulic characteristics of the shape and size of void spaces. Water can flow easily through certain rocks through the soil into the underground aquifer system, but water typically penetrates through fractures, cracks, and some other geological formations. Generally, there are three distinct types of geological formations of groundwater that determine the availability of groundwater resource, namely, aquifers, aquitard, and aquiclude.

#### **Figure 1.**

An aquifer is a highly permeable or porous saturated formation (conglomerate, sandstone, limestone, unconsolidated sand, gravels, fractured limestone, fractured basalt, etc.) that not only stores water but also provides adequate amounts of water and thus is considered as significant groundwater resources. The aquitard is a partially saturated formation (shale or clay) that allows water through it but does not provide enough available water than the aquifer. An aquiclude is an impermeable layer (clay) produces a considerable volume of water because of its high porosity but does not provide significant amount of water.

Groundwater passes from recharge zones to discharge zones along flow routes of variable lengths and comes in contact with surface water (SW) essentially at low elevated areas [3]. Surface water resources mostly depend on regional precipitation/ rainfall and it may be lost by infiltration through the streambed, layer of soil-moisture, and cracks or fractures to interacts with the groundwater system and the area of mixing of both known as the hyporheic zone [4]. The interaction of surface water and groundwater takes several forms in which if surface water moves toward the groundwater system, it is referred to as a losing stream whiles the other way round is called gaining stream [5].

The surface water (SW)-groundwater (GW) interactions in the hyporheic zone take place within the close-streambed sediments at few scales, which depend on the hydraulic-potential strengths and bed geometry [6]. In SW-GW interactions and hyporheic exchange, earlier studies used three types of scales such as sediment scale (<1 m), local scale (1–1 km), and catchment scale (>1 km) [7]. However, Todd and Mays [8] classified only two scales such as local scale and regional scale which are associated with small watershed and large watershed, respectively. Here, SW-GW interaction is associated with the direction of streamflow, shallow GW aquifer property, and local GW flow system.

The scope of this chapter is to discuss the vertical distribution of GW, types of the aquifer system, types of SW-GW interface, and SW-GW interaction at both local and regional scales. This chapter has been divided into four sections; (i) groundwater distribution and aquifer characteristics, (ii) the SW-GW interactions at local and regional scale, (iii) types of SW-GW interface, and (iv) the methods for investigation of SW-GW interactions and aquifer system.
