Patterns of Groundwater quality - In sandy aquifers under environmental pressure

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.