SUBSURFACE GROUNDWATER STORAGE

SUBSURFACE GROUNDWATER STORAGE

Groundwater sustainability is based on the rate of water removal, volume of water available (water in storage and rate of replenishment), and the ability of an aquifer to yield water (effective porosity). The hydraulic impact of water production is observed in declining hydraulic head and aquifer water levels. In confined aquifers with acceptable rates of groundwater production, water is removed and head declines, yet aquifers remain fully saturated and potentiometric surfaces remain above the stratigraphic tops of geologic units. Therefore, useable aquifer storage is the volume of water that can be removed while maintaining head above the stratigraphic top of the aquifer.

Specific storage (S_s) is the amount of water per unit volume of a saturated formation that is expelled from storage due to compressibility of the mineral skeleton and the pore water per unit change in head (Fetter, 1994). Accurate determination of specific storage requires a number of terms including density of water, gravitational acceleration, compressibility of the aquifer skeleton, compressibility of water, and average effective porosity. All terms are generally known except effective porosity. Effective porosity is that portion of the total void space of a porous material that is capable of transmitting water (Barcelona, 1984). One of the most accurate determinations of porosity is obtained from neutron/density geophysical logs. Two neutron/density logs were available from oil and gas test wells in the project area in Henry and Bullock Counties. However, only the Eutaw Formation, Tuscaloosa Group, and Lower Cretaceous were logged in the fresh-water section. Values were recorded for coarse-grained units with effective porosities identified by GSA Net Potential Productive Interval mapping.

The storage coefficient, or storativity (S) is the volume of water that a permeable unit will absorb or expel from storage per unit surface area per unit change in head (fig. 61). Therefore, storativity of a confined aquifer is the product of the specific storage and the aquifer thickness (b) (Fetter, 1994):

S = bS_s

When storativity is multiplied by the surface area overlying an aquifer and the average hydraulic head above the stratigraphic top of a confined aquifer, the product is the volume of available groundwater in storage in a confined aquifer (Fetter, 1994):

V_w = SA h

Table 23 shows measured and estimated effective porosity, aquifer thickness, storativity, and the volume of available groundwater in storage for major confined aquifers in the project area. Groundwater in storage for the Lower Cretaceous undifferentiated is included in table 23. Currently, Lower Cretaceous sediments are not developed as water sources in Alabama. However, evaluations of electric and geophysical logs and drill cutting descriptions in oil and gas test wells in the project area indicate that Lower Cretaceous sediments may have future potential as sources of fresh water. Total fresh groundwater in storage for the project area is given in table 23.

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SURFACE-WATER AVAILABILITY

In preparation for a statewide surface-water availibility assessment, the ADECA OWR, in partnership with the USGS developed a large data set of ungauged stream discharge estimates. ADECA OWR performed statistical analyses on the raw discharge data to provide flow-duration and low-flow characteristics and average daily monthly and annual flows.

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INSTREAM FLOW

Instream flows are often thought of as “minimum flows”. This is a misconception because minimum flows may not fully protect instream uses and values. Minimum flow regimes have led to the depletion and degradation of many rivers and streams. Minimum flow standards impair hydrological and ecosystem function because the natural flow variability component has been removed. In some cases, minimum flows actually become maximum flows in highly used, hydrologically altered systems because managed flows are rarely allowed to exceed the “minimum” limit (AWAWG, 2013).

Instream flow water rights have been established in other states to legally protect water levels for the conservation of aquatic habitat and biota. Through the implementation of instream flows, water resource managers can strive to achieve a flow regime that maintains natural processes necessary to support ecosystems while also balancing human use. In order to make appropriate management decisions regarding instream flow for the state of Alabama, several questions will need to be addressed: (1) how can this hydrologic regime be implemented within water resource management policies to provide for protected ecological functions and uses while also allowing development of water resources for human needs and economic activities for off-stream uses, and (2) how much ecological degradation are we willing to socially accept given a certain level of water resource development? Answers to these questions may be provided through the implementation of practical research to produce solutions to flow-related ecological issues (AWAWG, 2013).

Successful management of an instream flow regime requires that science-based procedures are applied not only in the initial planning stages of water resource management, but also in the research and application phases as well. The USGS is conducting research to update low-flow statistics at USGS stream gauges and regionalize selected low-flow characteristics for Alabama streams. The project is a joint effort between the USGS, the USDA NRCS, and Alabama state agencies. Historically, low-flow statistics, such as the annual minimum 7-day average flow that likely will occur, on average, once every 10 years (7Q10), have been used by water-resource managers as a threshold criterion for applying the chronic aquatic life criteria for determining waste-load allocations for point sources, total maximum daily loads (TMDL) for streams, and the quantity of water that can be safely withdrawn from a particular stream. It is critical to effectively measure and document base-flow data for use in updating low-flow frequency relations on a regular basis, preferably every 10 years, and especially after periods of extreme low flow, such as have occurred in the Southeast in recent years. Since low-flow statistics in Alabama have not been updated on a statewide base since 1990, this research is crucial to provide better documentation of flow characteristics for Alabama streams (USGS, 2014e).

For some states, the regulated riparian regime of permits and licenses is standard and requires adaptive elements for effective management of water use and supply across watersheds. Water legislation and implementation vary widely from state to state and few methods link flow regimes to maintaining functional stream ecology while also considering local water requirements. Federal environmental legislation such as the Clean Water Act and Endangered Species Act play a role in protecting instream flows in rivers, but only in an indirect manner. Certain state agencies in the southeastern U.S., including Alabama, have utilized the Public Trust Doctrine through state conservation agencies to protect instream flows, but the full extent of inter and intra-annual flow variability is not considered in these requirements.

The state of Alabama has no law prescribing instream flow standards. However, the ADCNR adopted an instream flow policy in 2012 under the Public Trust Doctrine for all flowing waters of the state. This policy was the first state agency step toward managing instream flow in a more comprehensive, ecologically protective manner in Alabama and will require further work on specific implementation details. It is partly based on the percentage-of-flow approach used in several states which serves as guidance in all negotiations with industries and other agencies with regard to protecting aquatic habitat, fish, and wildlife. Instream (conservation) flow regimes have been prescribed for some main river channels in Alabama by ADCNR through Federal Energy Regulatory Commission (FERC) negotiated site-specific flow requirements for large utility projects. The ADCNR is charged with the duty to protect, conserve, and increase the wildlife of the state (Code of Alabama, 1975, §9-2-2). Maintaining ecologically significant instream flows is fundamental to fulfilling the trustee resource conservation requirements of the ADCNR. The Public Trust Doctrine provides an indirect means of protecting flow-dependent fish and wildlife resources held in trust for the people of the State. But, while the public trust doctrine regarding water appears to be a legislative duty, other policies and laws involving water ownership need to be addressed to achieve balanced, natural flow variability in order to provide a holistic water management framework for the state (AWAWG, 2013).

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SUSTAINABLE YIELD

Generally, groundwater extraction regimes characterized by wells with adequate spacing, wells constructed in multiple aquifers, if available, and extraction rates that prevent excessive water level drawdown, will acquire acceptable levels of aquifer stress and will be sustainable for the long term (Cook and others, 2014). Aquifer stress areas in southeast Alabama are generally in and near population centers where water demand is high and where relatively large numbers of high capacity wells are extracting groundwater in close proximity (Cook and others, 2014). Evaluations of groundwater levels, drawdown, well spacing, and extraction rates for groundwater extraction regimes in 13 counties in southeast Alabama were evaluated during the southeast Alabama component of the GSA statewide groundwater assessment (Cook and others, 2014). Based on this evaluation, a number of areas in southeast Alabama have readily identifiable aquifer stress, yet no well or group of wells currently has an unacceptable level of stress (Cook and others, 2014).

In order to ascertain the sustainability of groundwater resources in a specified area, available volumes of groundwater of adequate quality must be compared to current groundwater use. As mentioned previously, current water use values are not available. Therefore, total volumes of available groundwater in subsurface storage and confined aquifer recharge were compared to 2005 water use values for the southeast region of the GSA statewide groundwater assessment (Cook and others, 2014). An exact comparison is not possible, since groundwater use data are compiled for geographic areas and are not available for specific aquifers. However, improved insights into groundwater availability and current groundwater production impacts can be developed by comparing available information. Unconfined or partially confined recharge was not included in the comparison, since water use from unconfined aquifers in southeast Alabama is relatively minimal. Also, groundwater use data includes all aquifers, which are compared to groundwater availability values for selected aquifers.

Total available groundwater in subsurface storage for all assessed confined aquifers (Lower Cretaceous, Coker, Eutaw/Gordo, Ripley, Clayton/Salt Mountain, and Nanafalia) is about 8.0 billion gallons and the Gordo, Ripley, Clayton, and Nanafalia aquifers are being replenished at a rate of 117.0 mgd. This is compared with total 2005 groundwater use for 13 counties in the assessment area, which is about 123 mgd. Therefore, when confined recharge rates for minor aquifers are considered, 2005 groundwater use is equivalent to confined recharge (Cook and others, 2014). Although the groundwater use and availability comparison from the southeast region of the GSA statewide groundwater assessment was for the entire southeast region, these data are applicable to the CPYRW.

DROUGHT IMPACTS

The Palmer Drought Severity Index (PDSI) was used to determine periods of drought in Southeast Alabama from 1929-2013. The PDSI is a tool developed by the NOAA to identify prolonged and abnormal moisture deficiency or excess. The PDSI is an important climatological tool for evaluating the scope, severity, and frequency of prolonged periods of abnormally dry or wet weather. It can be used to help delineate disaster areas and indicate the availability of water supplies for irrigation, reservoir levels, range conditions, adequacy of stock water, and potential intensity of forest fires. The PDSI calculations include the weekly precipitation totals, average temperature, division constants (water capacity of the soil), and previous history of indices. PDSI indices indicate general conditions; they do not indicate local variations caused by isolated rain. The equation for the PDSI index was empirically derived from the monthly temperature and precipitation scenarios of 13 instances of extreme drought in western Kansas and central Iowa by assigning an index value of -4 for these cases. Conversely, a +4 represents extremely wet conditions (NOAA, 2005). From these values, 7 categories of wet and dry conditions are defined (table 25).

Drought conditions were examined for the CPYRW, utilizing PDSI data from the CONUS [spell out conterminous or contiguous—or whatever is meant by this abbreviation—only one use] Alabama climate division 7 to estimate the total number of months of drought from 1930-2013 (fig. 64). Since 1930, there have been 36 occurrences of moderate drought; 19 occurrences of severe drought; and 5 occurrences of extreme drought in the CPYRW. Each period of drought varied from less than one year to more than seven years. Instances of sustained drought caused severe impacts on local agriculture and the general economy. The first instance of sustained drought (moderate and severe) occurred from 1930-1934. One of the worst sustained droughts ever recorded within the state of Alabama spanned from 1950-1963 statewide, and from 1950-1957 in the CPYRW (fig. 64). The drought of record occurred in 1954 with 2 months of moderate drought, 2 months of severe drought, and 5 months of extreme drought; and 1955 with 5 months of moderate drought, 3 months of severe drought, and 4 months of extreme drought (fig. 64). In 2000, the CPYRW experienced 4 months of extreme drought, 3 months of severe drought, and 2 months of moderate drought. A period of extended drought occurred from 2006 to 2012, excluding 2009, in which most of the drought months were severe. The worst conditions during this period occurred in 2011, with 1 month of moderate drought, 9 months of severe drought, and 2 months of extreme drought (fig. 64).

The GSA has a periodic monitoring well system that tracks groundwater levels in 369 wells and 49 spring discharges throughout the state (fig. 65). Some of the water sources in this program have been monitored for decades and reflect changing climatic conditions and water use patterns. There are 63 periodic monitoring wells within the CPYRW area. Recorded water levels in each well have been used to construct hydrographs (graph showing depth to water level relative to time) to assess climatic and water use impacts for most aquifers in the CPYRW.

The hydrograph for well A-9 in Coffee County shows declining water levels during the drought of 1990, when the water level dropped 10 ft (fig. 66). This well is 242 ft deep and is constructed in sand and limestone of the Clayton Formation of Paleocene Age.

The hydrograph for well M-5 in Covington County shows declining water levels during the drought of 2006-2007, when the water level dropped about 11 ft. Well M-5 is 170 ft deep, supplies water for an industrial plant, and is constructed in the Lisbon Formation of the middle Eocene age (fig. 67). Well B-8 in Dale County is an institutional supply well, 270 ft deep, and is constructed in the Tuscahoma Sand of the Paleocene age. The hydrograph shows drought impact during 1990, where the water level dropped about 10 ft and did not recover until 1991-1992 (fig. 68). Additional declines occurred during the 2007 drought, when the water level dropped about 10 ft and did not replenish until 2009 and declined again during the 2010 drought.

Well L-7 in Geneva County is an unused well, 322 ft deep, and is constructed in the Tallahatta and Lisbon Formations of Early and Middle Eocene Age. Since this well is not impacted by water production, water levels reflect climatic variation. Severe water level impacts occurred during the 2000 drought, when water levels dropped 11 ft (fig. 69).

Drought impacts are most severe in surface water bodies and shallow aquifers. The magnitude of groundwater declines caused by drought varies locally due to differences in groundwater conditions, water requirements for humans and the environment, and depth and hydraulic properties of aquifers. Wells in deeper, confined aquifers generally show minimal response to drought. The hydrographs shown above are from wells in relatively shallow aquifers where variation in water level caused by drought impacts are readily observed. Figure 70 shows an example of a deep, confined well with no significant effects from drought. This is a real-time well from Marion County that shows continuous water level measurements from 1952. The well is 520 ft deep and is constructed in the Pottsville Formation. Water levels in the well exhibit regular, seasonal fluctuations and minimal declines during periods of drought (fig. 70).

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