Patterns of groundwater quality in sandy aquifers under environmental pressure

Abstract

In exploring groundwater quality patterns in sandy phreatic aquifers this thesis follows three fundamental tracks, which are of a methodological, of a conceptual, and of a process based kind. First, in chapter 1 the three principle factors that together determine the groundwater quality distribution, which are flow, input, and geochemical processes, are introduced. Also the two perspectives of relating groundwater flow to groundwater quality are presented. The first perspective is in terms of future and past pathways of point source pollution, the second is of a spatiotemporal type, where the water quality in streams, wells, or wetlands is related to the historical landuse within their recharge areas. These concepts form the framework for the following chapters.

In chapter 2 groundwater flow is mapped in terms of the two mentioned perspectives. I show that some powerful 3D aspects of groundwater flow models are underexplored, and that this is due to (1) the lack of adequate visualization methods, and (2) the incompleteness of existing definitions of groundwater flow systems. Both problems are resolved by formalizing groundwater flow systems analysis in three dimensions and by the development of new mapping techniques. This is illustrated for the Hengelo area. Some implications are elaborated upon, both in terms of groundwater flow system and model scale as well as from a conceptual and spatial planning viewpoint.

However, such maps are of limited practical value when the uncertainty of groundwater flow is unknown. In chapter 3 it is shown that existing techniques for estimating uncertainty mainly focus on groundwater levels, whereas in relating groundwater quality to groundwater flow other characteristics of groundwater flow are important. A methodology was developed that assesses the uncertainty in groundwater flow system configuration, and is illustrated for the Salland area. The resulting maps of uncertainty due to anisotropy, drainage resistance, recharge, and drainage level are discussed.

In chapter 4 the often used age-depth relation, as presented in chapter 1, is extended towards an exhaustive framework for groundwater quality patterns. The concept of hydrogeochemical streamtubes is introduced, by which water quality boundaries and changes in their location, as observed in the Salland multilevel well section, are related to flow, input, and processes. Observed patterns are shown to be better explained in this way, especially as the framework presented takes into account groundwater flow system size, position within the groundwater flow system, land use, and land use scale.

In chapter 5 both the sources and controls of trace and major elements are explained using a sequential approach that addresses all known important controls and sources for all trace elements of adequate analytical quality. Advances were made by following two new approaches. First bulk geochemical soil data is used to better explain observed concentrations in terms of codissolution and coprecipitation, and secondly the SEQSSI-approach (sorption equilibrium through steady state input), which explains the behaviour and concentrations of a large set of trace elements is presented.

The thesis concludes with a synthesis in which practical spin-off, in terms of background values, modelling, spatial planning, and monitoring, are further elaborated upon.