“An evaluation of the removal of salt water from the Charles River”, Metropolitan District Commission
Large amounts of seawater in the lower portion of the Charles River basin cause a stable stratification which is difficult to disturb. By late summer, and in dry years, a saltwater layer extends all the way to Watertown dam. The density of salt water is higher than fresh water, causing salt water to remain on the bottom. This layering prevents vertical mixing of oxygen-rich surface waters with oxygen-poor bottom waters, resulting in anaerobic bottom muds and steady production of hydrogen sulfide.
Bottom muds throughout most of the basin contain significant amounts of metals, pesticides, oil and grease, and other pollutants. Benthic organisms are limited, and the muds exert a significant oxygen demand. Surveys have indicated that 21 species of fish reside in the basin at different times during the year. However, spawning cannot occur in toxic muds.
In the lower basin, downstream of Boston University Bridge, the stratification is currently strong and stable enough so that, although malodorous gases are generated, they are not brought to the surface and liberated. Upstream of the bridge, on the other hand, the small amount of diluted seawater which reaches that part of the basin results in a weak stratification. This is easily disturbed and periodically releases noticeable quantities of malodorous gases.
Water quality will be improved by removing the stratification. Possible methods include:
1. curtailing intrusion of salt from the harbor (currently, most salt water enters during lock operations; if this intrusion were reduced or curtailed, the natural stream-flow would in time reduce the salt content of the basin)
2. vertically mixing the contents of the basin, to destroy the stratification maintained by residual salt content and by any vertical temperature gradients.
With the new dam in operation, the rate of salt intrusion from the harbor will be reduced by 80 to 90 percent, principally because of the smaller locks (pleasure boats will be the principal users of the new dam). However, this reduction by itself will not ensure consistent quality control, which vertical mixing capability will provide. Although stratification in the upper basin will be almost completely eliminated, it will still occur in the lower basin. In summer, when seawater intrusion rates are greatest because of heavy lock traffic, the decreased freshwater in-flow will not be sufficient to purge the basin entirely of all the sea-water, as it intrudes. Temperature stratification downstream of Boston University Bridge will also occur.
In the lower basin, the present strong stratification, which traps and contains malodorous gases, will be replaced by a weaker, less stable stratification, sufficient to promote anoxic and odor-causing conditions much of the time but too weak to prevent occasional release of these odors.
Satisfactory conditions in the basin can be maintained, as long as aerobic conditions are maintained (e.g., by mixing the waters as necessary, to overcome stratification and oxygen deficiency). If the basin is prevented from stratifying, natural reaeration will maintain aerobic conditions, and noxious odors will not be generated.
The most satisfactory way to prevent stratification, or to destratify, is by means of a simple air mixing system. Air fed to diffusers located at low points in the basin will pump bottom waters to the surface. Field tests indicate that the air/water flow can be regulated, so that it will not disturb or resuspend a significant amount of bottom muds. We propose that diffusers be installed at five locations between Boston University Bridge and the new Charles River dam.
The current saltwater intrusion provides a continually varying salinity, which appears to inhibit the severity of algal blooms. Reduced saltwater intrusion, with operation of the new dam, might enhance conditions favorable to freshwater algal blooms. Such blooms may be controllable, by deliberately increasing the salinity of the basin waters (i.e., admitting salt water).
The recommendations of this study may increase the number of fish, but not significantly.
Basin muds are of extremely poor quality. However, dredging the entire basin bottom would be too costly (both in dollars and environmental effects). Creation of an aerobic water layer, in contact with them, will improve their quality.
If a no-action program were adopted, stratification would occur and would be subjected to uncontrolled disruption. Anaerobic conditions would exist in parts of the basin, and odors would continue to be produced. Prior to full service of the new dam, these odors would continue to be concentrated in the upper part of the basin. After the new dam was operable, the upper basin would probably be odor free. However, odors could be anticipated in the lower basin, and algal blooms would also be a greater possibility.
It is recommended that a permanent destratification system be installed for use by the time the new dam is placed in full service. This system will allow the basin to be destratified or mixed, whenever necessary.
Construction of a permanent destratification facility will (1) improve the aquatic environment of the basin by maintaining aerobic conditions throughout, (2) prevent stratification resulting from temperature or salt water, and (3) result in an odor-free discharge from the flood control pumps at the new dam. Moreover, the entrance of seawater because of lock operations or leakage will not cause any problem, because the saltwater wedge will be homogenized with the fresh water. Stabilization of bottom sediments will be promoted, and overturns of the lower basin waters will not result in odors.
Destratification facilities will consist of compressors, located in the upper gate house at the old dam, supplying three air diffusers. One air diffuser will be located upstream of Longfellow Bridge, one downstream of Longfellow Bridge, and one in the area between the new and old dams. Compressors will also be located in the Fens gate house, supplying two air diffusers. One air diffuser will be located downstream of Harvard Bridge and one upstream toward Boston University Bridge.
Mixing equipment will be started each year at the breakup of ice cover, when the volume of salt water in the basin is at a minimum. Several days of operation will mix the basin thoroughly and ensure homogeneous, aerobic conditions from top to bottom. From breakup of ice cover through October, mixers adjacent to the old dam will operate continuously; the other deep parts of the basin will be monitored for dissolved oxygen. If oxygen is deficient in any of these areas, mixing equipment may be turned on for a few days until aerobic conditions are restored. Dissolved oxygen monitoring is quite simple, consisting of lowering a probe to the appropriate point in the water and reading a meter attached to the probe. (Sampling or laboratory testing is not required.)
Estimated total cost of construction, including engineering and contingencies, is $500,000. Estimated annual operation and maintenance cost is $25,000 (June 1976 dollars).