Part I: Geology
By Michael Schnieders, PG, PH-GW
We face many challenges to find and deliver water of acceptable quality and quantity to our customers. Part of this task is new resource identification and development, while some is the ongoing maintenance of existing well systems.
As the infrastructure in this country ages, land resources become less available, and the understanding of earth processes and anthropogenic influence grows—so do our challenges.
In our work to better respond and extend the operational life cycle of potable well systems, we focus on three core sciences: geology, chemistry, and microbiology.
The skillful contractor has likely adapted their understanding of these sciences into their craft, often through practices rooted in one of these governing fields handed down overtime although not necessarily labeled as such.
This article, the first in a three-part series, will look at geology and the role it plays in wellfield maintenance and operation.
What Is Geology?
Geology is defined as the science that focuses on the earth’s physical structure and the processes that act on it. As such, geology plays an integral role in the work that we, as an industry, perform in the search for and employment of groundwater.
The geologic setting is a broader term to include not only the rock types present in a given locale, but the processes that have and continue to occur both above and below ground within the target area.
The rock types present govern the occurrence and amount of water, dictating the availability and sustainability of the resource. Furthermore, they significantly influence the water quality, both initially as the resource is explored, and later as the resource is utilized.
Two concepts within this topic are permeability and porosity. Permeability is the measure at which fluid moves through a porous substance. Porosity is the measure of the amount of open space or pores within a medium.
The two terms are often used hand in hand, but do not necessarily go together. Permeability is more a function of the movement of water while porosity is more a function of the structure of the rock type.
Secondary porosity relates to the interconnectedness of pore space, generally enhancing porosity. Loose compaction of sediments, fractures and crevices, or dissolution within the aquifer can cause secondary porosity.
Hydraulic conductivity (K) is defined as the ease of which water moves through pore space and fractures. Hydraulic conductivity represents permeability and the degree of saturation.
A variety of factors can impact groundwater movement and hydraulic conductivity. The type of sediments that comprise the rock unit, structural influences on the unit, geochemical reactions that occurred at the time of formation or over time with groundwater movement, and geomorphological processes which have shaped the units are just a few of the natural factors.
We cannot forget anthropogenic factors either, such as the construction of wells, mining, and alterations of recharge areas to name just a few.
Recharge is the process in which groundwater aquifers are refilled or replenished through a process called infiltration. Rates of recharge are dependent on the geology, anthropogenic influences (including use), and climate.
Artificial recharge is the process in which wells or basins are used to enhance recharge rates in targeted aquifers. Artificial recharge has become a popular process in which secondary water sources are used to recharge depleted aquifer systems or to bank water for future usage.
While beneficial to restoring our aquifers, it is important to understand that recharge is a major source of natural and anthropogenic contamination.
Influences of an Aquifer
Aquifer influence on produced water quality is generally viewed in a regulatory sense. Quite simply, does the water meet the criteria outlined in the National Primary and Secondary Drinking Water Regulations?
Of the 103 chemical and biological constituents listed in the National Primary and Secondary Drinking Water Regulations, many are anthropogenic or a result of contamination, but some are directly related to the geology present.
Within the Primary Regulations, arsenic, selenium, thallium, turbidity, and uranium are directly related to the lithology present within the aquifer system. Many of the secondary constituents relate to the aquifer composition such as chlorides, copper, iron, manganese, sulfide, zinc, fluoride, and total dissolved solids. Some of the constituents reflect the geologic setting and recharge, some varying regionally such as nitrates.
Beyond the composition of the producing formation, the means of formation and physical characteristics can influence many aspects of well construction and operation.
Unconsolidated alluvial aquifer formations comprised of well-sorted materials such as coarse sand and gravel generally have higher permeability and hydraulic conductivity. While often seen as good producers, these aquifer systems can influence the mechanical, chemical, and biological qualities of produced water.
Generally, alluvial aquifers have higher levels of conductivity, elevated oxidation-reduction potential, elevated iron and manganese, and varying levels of microbial activity.
Unconsolidated alluvial aquifers can be strong producers and are commonly utilized as municipal water sources. As such, alluvial aquifers commonly suffer from iron fouling as well as biofouling, requiring periodic maintenance.
In addition to chemical and biological fouling, these aquifers can suffer from the mobilization of fine sediment toward the borehole over time, mechanically clogging pore space in the near-well aquifer interface zone and requiring redevelopment efforts.
Consolidated aquifers, such as limestone and sandstone, are porous rock types in which the grains are cemented to each other. Consolidated aquifers, which often provide a high degree of water quality, suffer from the development of mineral scale in reflection of their mineral content.
The mineralogy present within these aquifer systems commonly impart calcium and magnesium, influencing hardness, alkalinity, and pH of the produced water.
Fractured rock aquifers occur within solid rock units that have developed fractures, joints, cracks, or dissolution caverns in which water can move. Fractured rock aquifers, such as basalt or granite, can influence water quality with iron, manganese, and silica concentrations impacting scale formation and corrosion rates.
Fractured rock and consolidated rock aquifer units are commonly completed with a method referred to as open-hole completion. Unfortunately, this means of design severely hampers mechanical cleaning of the well.
Mechanical cleaning utilizing jetting, surge block or swab tools, or brushing is a common means of pre-treatment as well as chemical agitation. As both aquifer types can suffer from mineral scale accumulation and biofouling, the limits of this design hampers effective rehabilitation of the well. Similarly, the design can limit development efforts, both initially and maintenance driven.
Goals of Construction
While very similar at times, each of these types of aquifers can be vastly different, impacting the construction and use of the well over its lifetime. Initial well design goals should follow historical as well as current concerns and make a best effort toward addressing future challenges.
With the geology in mind, well design goals should include:
- Construct to limit potential of surface influences.
- Construct to limit aquifer interaction.
- Construct with awareness of current and future potential contaminant sources.
- Design beyond minimum standards specific to the formation and aquifer vulnerability.
- Design and construct with access and future maintenance in mind.
The fifth goal is intended to look into the future, based on the information known regarding the producing aquifer and historical success and failures of wells completed within the same unit. It is included with the intention of imparting a maintenance mindset on the well owner. Not one geologic unit, completion depth, or aquifer type is immune to fouling and the need for periodic maintenance.
Physical observations of the well and well setting remain important tools. Subtle changes at the surface may telegraph major events occurring downhole. Evaluate the land surface and keep apprised of regional concerns as they develop—and, most importantly, before they impact your well.
Annual pumping tests to monitor the well’s capacity and efficiency are important indicators of the well’s health. Tracking the occurrence of particulate, whether it is fine sand or corrosion byproducts, is another important means of identifying problems early when they are at a more manageable state.
With geology, it is important that we do not dismiss the lessons learned in a specific region or aquifer.
If an area is notorious for hard water, with high amounts of calcium and magnesium present, it is likely it will have a propensity for carbonate scale development and require periodic chemical and mechanical cleaning. As such, we should use that information to design
and construct a well of less reactive materials to withstand the cleaning efforts while also being accessible to allow for the use of different methodologies.
In discussing chemistry and microbiology in the forthcoming articles, my colleagues will elaborate on the chemical and biological mechanisms that can impact produced water quality and quantity. These mechanisms are derived from the geology and geologic setting, influencing the well’s operational life just as much as the means of construction.
Michael Schnieders, PG, PH-GW, is the president and principal hydrogeologist at Water Systems Engineering Inc. in Ottawa, Kansas. Schnieders was the 2017 NGWA McEllhiney Lecturer in Water Well Technology. He has an extensive background in groundwater geochemistry, geomicrobiology, and water resource investigation and management. He can be reached at email@example.com.