Another method for reducing Cryptosporidium source concentrations is to move the intake within the same source. This could involve relocating an intake within a source or changing the depth from which the intake draws.
LT2ESWTR Toolbox Guidance Manual 3-3 April 2010
3.3.1 Applicability
Relocating an intake can be a good strategy if an obvious source of Cryptosporidium is present which can easily be avoided by moving the location of the intake. One example of such a situation is if an intake could be moved upstream of a municipal wastewater discharge in a river, where it had previously been located downstream of the discharge.
3.3.1.1 Advantages and Disadvantages
One advantage of moving the location of an intake is its potentially low relative cost, if the distance the intake must be moved is relatively short. This option could be particularly attractive if an existing intake structure can be used to withdraw water from a different depth, resulting in decreased Cryptosporidium concentrations.
Disadvantages could include significant amounts of excavation and piping, as well as additional pumping if the intake must be relocated a considerable distance. Also, altering the intake may not bring the desired reduction or provide any additional protection against future increases or spikes in Cryptosporidium concentration.
3.3.2 Reservoirs and Lakes
Several variables can affect the concentration of Cryptosporidium at a particular location in a reservoir or lake, including the intake depth, the way in which the lake mixes, the thermal properties of the lake, and the proximity of the intake to streams and other discharges. It is recommended that a water system develop an SOP for water withdrawal based on the specific conditions of the waterbody being used as the source.
3.3.2.1 Depth
The intake depth can significantly change the quality of the water being drawn and used. In general, shallow intakes are more likely to draw water exposed to recreational activity and surface water runoff. Deeper intake locations are often more protected from sources of
Cryptosporidium, unless an intake location is so deep that it draws water containing re-
suspended material from the lake or reservoir bottom. Water systems are often well-advised to draw water from intermediate depths, where they can avoid higher oocyst concentrations that may exist near the lake or reservoir surface, and also avoid particles that may be stirred up near the bottom.
3.3.2.2 Stratification and Mixing
Another factor that can affect the depth profile of Cryptosporidium in a lake or reservoir is the amount of stratification or mixing present. Larger lakes and reservoirs often stratify, especially in the summer months, forming a hypolimnion (a cold lower layer) and an epilimnion (a warm upper layer) separated by a thermocline. There is very little mixing between these layers when a lake is strongly stratified. Particles may settle through the layers, but there is little other mixing. The epilimnion is often well mixed because of the mixing action of wind. Therefore, it is likely that Cryptosporidium may be present at uniform concentrations throughout the epilimnion.
Cryptosporidium oocysts that have attached to particles and settled will have a concentration
gradient in the hypolimnion. The shape of any concentration gradient will depend on local conditions such as temperature, stream inflows, and particle settling rates. Lakes or reservoirs that are strongly stratified and have a high input of organics can often develop anoxia in the hypolimnion. Therefore, all water quality parameters should be considered before determining the depth from which to draw the water. Extremely high withdrawal rates may provide enough energy to overcome stratification and draw from the layer outside of where the intake is located.
3.3.2.3 Proximity to Inflows
The proximity of the intake to stream inflows may affect the quality of the intake. Streams carrying agricultural or urban runoff can cause water quality degradation if located too close to a source water intake. States et al. (1998) reported an increase in Cryptosporidium concentrations with wet weather events, particularly as the sampling location became closer to the contamination source. Kortmann (2000) reported a system substantially reduced coliform bacteria in their source water by moving their intake further away from a stream which drained an agricultural area and by installing an artificial partition in the reservoir to limit the exchange of water between the stream input and the rest of the lake.
3.3.3 Streams and Rivers
There are several factors to consider when deciding where to locate an intake on a river including depth, flow hydraulics, seasonal effects, and upstream sources of contamination.
3.3.3.1 Depth
Depth is not as likely to affect Cryptosporidium concentrations in small rivers and streams as it is in lakes and reservoirs. Fast moving or shallow streams are likely to be fairly mixed across all depths. In contrast, deeper and slower moving rivers may be less mixed and may show some concentration gradient of Cryptosporidium with unattached oocysts being greater near the surface and oocysts attached to particles being greater near the bottom. In rivers and streams, intakes located near the bottom are more likely to draw sediment and other particles resuspended from the bottom.
LT2ESWTR Toolbox Guidance Manual 3-5 April 2010
3.3.3.2 Flow and River Hydraulics
Hydraulics of the river and the flow around the intake are extremely important in determining the quality of water that enters the system. In general, portions of a stream or river with lower velocities and less turbulence will contain less sediment and possibly less
Cryptosporidium oocysts. Care should also be taken to make sure that the design of the intake
does not cause turbulence which might stir up sediments.
3.3.3.3 Upstream Sources of Contamination
Any potential sources of contamination upstream of a new intake should be identified and their impact considered with respect to both biological and chemical contamination.
Contaminant sources of particular concern for Cryptosporidium include animal feeding operations and sewage outfalls. If an intake cannot be located upstream of such a source, then locating it as far as possible downstream to allow time for particles to settle may be the next best alternative. Analyses of the vulnerability of a stream source should be made on a regular basis. Any changes in the vulnerability of a source to Cryptosporidium or other contaminants should be reported to the primacy agency.
3.3.3.4 Seasonal Effects
Cryptosporidium concentrations tend to be higher during runoff events, particularly in the
spring. Although it is probably not feasible to cease withdrawals during such incidents, it may be possible to alter flow rates and coagulant doses to offset the effect of such events.