Groundwater Resources in Wisconsin

Introduction

Groundwater is an important source for the people of state of Wisconsin, despite the availability of great amount of surface fresh water. Figure 1 shows the water resources in state of Wisconsin.

About three quarter of the population and especially who are living on rural areas rely on groundwater as a source for drinking purposes. Around 1,207,748,500 m³/year (815,000,000 gallon/day) of groundwater is pumped and used for various purposes.

The source of groundwater is the precipitation and surface runoff. Wisconsin receives about 0.79 meters (~ 31 inches) of precipitation per year. Less than 0.25 meters (10 inches) infiltrate down to the aquifer systems.

Groundwater Aquifers in Wisconsin

In the state of Wisconsin there are four major aquifers: the sand and gravel aquifer, the dolomite aquifer, sandstone and dolomite aquifer, and the crystalline bedrock aquifer.

1          Surficial Aquifer System

The sand and gravel aquifer is the most widespread, extensively used and covering most of the territory of the state except the south west part (Fig.2). It cover approximately 70% of the state.

It consists of stratified sand and gravel deposited from glacial melt water (glacial drift) or in river floodplain. The aquifer materials deposited within the ice age (Pleistocene Epoch). The sand and gravel outwash plain is considered shallow and one of the best aquifer in the state. Because the aquifer is shallow is it vulnerable for contamination.

The well yields penetrating this aquifer are variable and depend on the water bearing formation thickness and permeability of the strata.

2          Eastern Dolomite Aquifer

The aquifer consists of carbonate dolomite materials with some shale and covering only east of Wisconsin (Fig.3). The rock formation was deposited in the Lower Paleozoic era in the Silurian period. The Silurian period begins 435 million years ago and spans 23 million years. The dolomite CaMg(CO3)2 is a common sedimentary rock-forming mineral that can be found in massive beds all over the world and are quite common in sedimentary rock sequences.

The aquifer is mainly unconfined and the subsurface flow is dominated by the rock fracture. The Maquoka shale layer beneath the aquifer is considered impermeable and act as a barrier between the dolomite aquifer and the sandstone and dolomite aquifer.

3          The Sandstone and Dolomite Aquifer

The aquifer consists of sequences of sandstone and dolomite with some shale. It is found in cental and south of Wisconsin (Fig.4). The subsurface flow moves in the porous media in the sandstone and between the rock fractures in the dolomite. The aquifer is an excellent source for groundwater. In eastern Wisconsin the aquifer is confined as it lies below the Silurian dolomite aquifer and at other places it lies below the surficial aquifer. The rock formation was deposited in the Lower Paleozoic era in the Ordovician and Cambrian periods. The Ordovician period begins 492 million years ago and spans 57 million years. The Cambrian period begins 570 million years ago and spans 78 million years. It is the earliest period of the Palaeozoic era. Well yields penetrating this aquifer are high.

 4          Crystalline Bedrock Aquifer

The aquifer is composed of a variety of weathered rock that formed in the Precambrian Era. The Era had begun 5 billions to 570 million years ago. Precambrian time includes 80% of the earth's history. The aquifer consists of weathered igneous granite and metamorphic quartzite. It underlies the entire Wisconsin and the surficial aquifer in the north central part of the state. Groundwater originates from fractures that exist in the rock and its yield is very low. It is found in the central and north of Wisconsin below the surficial aquifer (Fig.5)

Groundwater Wells in Wisconsin

In state of Wisconsin the number of wells estimated in 1996 to be 8,000 high capacity wells. High capacity wells are wells where the yield estimated to be 16 m³/h.  Approximately 1,700 are public water wells, 3,800 irrigation wells, 800 industrial wells, and the rest serves various communities and facilities such as schools, commercials and wastewater treatment plants.

Almost 5,550 wells with their chemical data downloaded from the USGS web page. The well classified based on the aquifer they penetrated (Fig. 6).  

Tritium Concentration as an evidence of Groundwater Recharge

There are many technical methods to estimate the recharge and test if the aquifer is rechargeable or not. Based on various studies conducted on groundwater resources, it shows that the withdrawal from the aquifers is much more exceeds the natural recharge.

Figure six shows that some groundwater wells is tritiated, means local recharge take place. 

Tritium (T) is a radioactive isotope of hydrogen (3H), has a half-life of 12.43 ± 0.03 years and decays by the emission of β^- particles with a maximum energy of 18 Kev.

Its natural abundance is usually expressed in Tritium Units (TU) (1 TU = 1 ³H/10¹⁸ hydrogen atom). Tritium is produced in the atmosphere by the interaction of cosmic ray produced particles with the nuclei of atmospheric gases, principally by proton and neutron induced reactions.

                 n + ¹⁴N ----------- ¹²C + ³H      II-3

When the tritium decays it changes to helium by the reaction:

                ³H ---------------- ³He + β        II-4

Tritium is produced in the upper stratosphere and is oxidized to H₂O and gradually transported into the lower troposphere to reach the Earth's surface as tritiated water in rainwater, snow and atmospheric moisture. Due to thermonuclear testing since 1953, the tritium content of the atmosphere has increased greatly. Maximum levels of this tracer recorded up to 10,000 T.U. in Ottawa (Canada) in the northern Hemisphere, following extensive testing in 1961-1962. Tritium concentration has decreased since in the atmosphere since this era. Tritium remains a good indicator of the presence of recently infiltrated water. Tritium can be used to trace the movement of meteoric waters throughout the hydrological cycle. The tritium content has been utilized in different hydrological applications, successful dating of groundwater e.g. measuring the degree of mixing of two or more water sources, and other applications.

Groundwater Quality

1. Nitrate in Groundwater (Under Constructions)

2. Radioactivity in Groundwater (Under Constructions)

3. Trace Elements in Groundwater (Arsenic) (Under Constructions)

Under Constructions

Last updated on June 16, 2003