The source of water for an aquaculture facility is very important and sources are manifold. Water sources can be from underground (groundwater), on the ground (surface waters), or above the ground (rain). Each source has unique water-quality parameters associated with it. An evaluation of a potential site for aquaculture should at least include a basic testing for the following water quality parameters as shown in Table 1. If a sample were taken from a natural surface water and evaporated it would contain organic matter in addition to the usual salts. Organic matter consists of living material, it excretions or decomposing dead material. This is usually only a small percentage of a residue. Organic matter can have a great effect on water chemistry by requiring oxygen for living processes as the complex organic matter is built up from and torn down into simple compounds.

A more extended summary of the sources of water and associated parameters relevant to aquaculture is presented in Table 2. Recommendations for safe reuse of wastewater in aquaculture is given in Table 3. This basic information given here is intended to provide an overview of water sources and a framework from which to judge their suitability for aquaculture.

Water Sources and Some Associated Water-Quality Parameters

GROUNDWATER

Groundwater refers to water that is contained in subsurface geological formations. It is brought to the surface either by pumping from a well or flowing naturally from a spring or artesian well. Water has a mass of about 1000-1028 kg/m3 (62-64 lb/ft3) depending on salinity and temperature. To move a large mass of water needed for fish culture any great height or distance requires energy. Water sources that are located deep in the earth or at great distance from the farm will always be more costly than water that is free flowing or shallow (2). The impact that this will have on the economic viability of the farm depends on many factors related to species and site selection.

Because groundwater is contained far from the surface and can remain underground for millions of years, the water takes on characteristics that are related to the rock formations that the water is in contact with. Alkalinity, hardness, pH, dissolved minerals, and dissolved gasses in groundwater can all be at very different levels from water that is in contact with the atmosphere. Oxygen and biological processes are limited underground, so ground water typically contains few pathogens and little oxygen, although carbon dioxide and argon gasses can be supersaturated.

Groundwater temperatures below 10 m (33 ft) are quite stable in a given location relative to surface waters. Temperature is moderated by the thermal mass of the earth, so groundwater tends to have a relatively minor seasonal temperature change. Groundwater varies with latitude, being warmer near the equator and colder away from the equator. In general, the average annual temperature of groundwater is a degree or two (°C) higher than the mean annual air temperature. In addition, groundwater temperature increases 1-5 °C (average about 2.5'C) per 100 m depth (4). Higher increases may be due to local geothermal activity. If the groundwater is in proximity to volcanic activity, then it may be naturally heated by a geothermal source.

Groundwater is usually fresh, but saltwater aquifers also exist. Saltwater aquifers are common near the coasts, but can also extend for hundreds or thousands of miles inland. The salinity of the groundwater is variable. Discharge into freshwater canals or streams may be problematic if the site is located very far inland or in agricultural areas.

SURFACE WATER

Surface waters include streams, rivers, canals, ponds, lakes, seas, and oceans. Because they are exposed to the atmosphere and typically support diverse and abundant biological ecosystems, surface waters have different characteristics from groundwater. Surface waters can be fed either by rain or groundwater or both. Water quality parameters such as alkalinity, hardness, pH, dissolved minerals, and other factors will be somewhat dependent on the source. In addition, biological processes tend to change water quality and add competing organisms, pathogens, and predators. Biological processes tend to add acids to water, there by lowering pH and depleting alkalinity. They also tend to reduce dissolved compounds in water and lower the oxidative-reductive potential (ORP) of the water. Water temperature follows seasonal and local weather patterns, therefore surface water is more variable than groundwater.

RAIN

Rain is not usually a reliable source of water for aquaculture; however, in certain regions it can be. Rainwater may also be a useable supply for makeup in recirculation systems or other systems where a continuous high volume supply is not needed. Since rainwater contains almost no buffer and can be affected by airborne pollution, careful consideration must be given to its quality and dependability as a primary source of supply. In some cases, it may help reduce demand on other sources.

MUNICIPAL WATER SOURCES

Municipal water can originate from groundwater, surface water, or rainwater sources. While it may be convenient to simply hook up to a municipal-water source, there are two drawbacks. First of all, municipal water is treated to make it safe for human consumption. Typically, this includes treatment with strong oxidizers, such as chlorine or ozone, `to kill human pathogens. These compounds are lethal to fish and need to be removed or treated to make the water safe for fish. Second, municipal water is expensive and a limited resource in most areas. The use of municipal water for aquaculture is largely limited to small-scale experimental systems, holding for live retail sales, or as emergency makeup for re-circulation systems.

NATURAL OR REHYDRATED SEAWATER

Small amounts of seawater can be made from salt mixtures and freshwater. This might be sufficient for inland holding facilities, aquariums, or small research re-circulation systems, but due to the expense and labour needed to prepare seawater, high-flow production aquaculture will likely need a large body of natural saltwater nearby. The advantage of making up seawater from salts is that very clean water can be made. This might be important for algae, zooplankton, egg, and larval culture, no matter where the facility is located.

RECIRCULATION AS A SOURCE

Just as recycling of paper and plastic can reduce the use of trees and petroleum resources, water re-circulation technologies can reduce the demand on the water supply. Water re-circulation can increase the flow to the culture units by 10-1000 times the flow of new water (9). The downside is the high capital cost of the treatment components.

Energy requirements may be greater or lesser than a single-pass system, depending on the amount of water that needs to be pumped in each system and the need for temperature adjustment. Temperature and other water quality parameters can be adjusted in a re-circulation system to suit the species and life stage of interest. This is a major advantage and may or may not offset high capital and energy costs and may offer higher growth rates. This advantage is particularly important with photoperiod and temperature-manipulated broodstock, egg incubation, and larval rearing.

Compiled after:
MICHAEL B. RUST, JOHN COLT
Northwest Fisheries Science Center Seattle, Washington
In:
Encyclopedia of Aquaculture, 2000.