RICHMOND MILLS
The taking of water from Hemlock Lake Outlet at Richmond Mills was considered as early as 1890 as a source of increased supply. Richmond Mills is located three miles downstream from Hemlock village. The drainage area is 81.8 square miles or 6.2 square miles greater than at Hemlock village, and a dam at Richmond Mills if built at elevation 911.3 (USGS) or to the same elevation as proposed for Hemlock Lake in conjunction with the Hemlock project, would provide 2 billion gallons additional storage, would eliminate the 6-foot brick tunnel and shorten the conduit lengths from Overflow No. 1 to Rochester by about 10,000 feet, and would provide a reservoir with two basins, as in connection with the proposed Hemlock project.
An inspection was made to determine the possibility of finding a suitable dam site at Richmond Mills. The bottom of the stream valley is flat and at about elevation 316.7 (825.0, USGS). No evidence was found of rock either in the stream bed or in the sides of the valley. The valley side banks are abrupt and are composed entirely of coarse sand and gravel interspersed with cobblestone. These conditions are highly unfavorable for construction of a water-tight dam.
A dam at this location with flow line elevation 403.0, as proposed in connection with the Hemlock project, would have a height from stream bed to spillway of about 86 feet, and even if rock foundation was available it would cost several times as much as a new dam at Hemlock village. In addition the drainage from the village of Hemlock would pass directly into the head of the lower basin of the reservoir, while in conjunction of the Hemlock project this drainage all passes into Hemlock Outlet downstream from the reservoir.
In spite of its attractiveness in other respects, further consideration has not been given to this project because of poor foundation and bank condition, lack of rock for spillway and danger of pollution.
HONEOYE LAKE
Honeoye Lake is at elevation roundly 100 lower than Hemlock Lake. It has been proposed to construct a dam on Honeoye Creek about 8.5 miles downstream from Hemlock Lake. The elevation of Honeoye Creek valley bottom at this location is about 275.0. Conduits nos. 2 and 3 follow this valley and the elevation of the top of Conduit No. 2 at air valve no. 57, near the proposed location of the dam, is 364.3 and the hydraulic grade line 386.4. The highest dam proposed at this location has flow line elevation 322.0 and no rock was found by borings at the proposed dam site.
A lower dam of course could be constructed, and was later proposed, but it would obviously be necessary either to pump the water or else to build entirely new supply conduits leading from this point to Rush Reservoir. In addition it has been suggested that the water should be filtered. The construction would involve the use of much larger quantities of defense materials than the Hemlock project and, furthermore, if this site is to be used to supplement the Hemlock supply, then various steps must be carried out to put the Hemlock system in condition for continued use.
The City of Rochester now has permission to use water from Honeoye Creek. If used to supplement the Hemlock Lake supply, then some of the steps involved in the first stage of the Hemlock project must be taken in any event to restore the Hemlock system to first-class condition. Since an adequate supply for the present and near future can be obtained from Hemlock Lake in conjunction with diversion from adjacent areas, it seem advisable that the Hemlock project should be developed first and that the taking of water from Honeoye Outlet should be deferred until some future time, if and when needed.
GROUND-WATER
The ground-water supply per square mile of area tributary to an aquifer is limited in three ways:
1. It cannot exceed the fraction of rain which enters the soil surface as infiltration during rain.
2. The soil can hold part of the water entering it as capillary moisture. This water does not pass downward to the water-table but is mostly absorbed by vegetation and returned to the air. Hence the ground-water supply cannot exceed the residue of infiltration after evaporation and transpiration by vegetation have taken their toll.
3. Water proceeds downward through the soil to a water-table only during rain occurring at times when the soil is filled with moisture to capillary saturation. Replenishment of aquifer occurs at irregular intervals and in limited amount.
Viewed in another way, total runoff is made up of surface runoff and ground-water outflow. The ground-water outflow represents the fraction of the total runoff available as ground-water supply. It is seldom that it can all be utilized. In the region around Hemlock Lake, for example, the total runoff is equivalent to about 12 inches depth of water per year, of which at least two-thirds to three-fourths is surface runoff, leaving the equivalent of only 3 to 4 inches depth to supply aquifers. In the immediate vicinity of Rochester there are local sandy areas where nearly the entire rainfall enters the soil as infiltration and nearly the entire runoff of streams like Irondequoit Creek is derived from ground-water flow. An aquifer consisting, for example, of buried deposits of sand or gravel fed by a tributary area of the first type, would require three to four times as many square miles of tributary area to yield 1 mgd as an area of the second type with the underground aquifer precisely the same in both cases, while the latter would require a drainage area tributary to the aquifer about the same as that required to provide an equal supply from natural runoff.
A ground-water supply to be adequate and dependable requires the concurrent conditions of suitable underground beds of porous material, which act as a storage reservoir, and a sufficiently permeable surface and sufficiently large tributary drainage area to replenish the aquifer. This combination of conditions is relatively infrequent.
In addition, ground-water, as a rule, is harder than surface water, although a high degree of purity from a bacteriological viewpoint.
There is a not uncommon misunderstanding regarding ground-water arising from the failure to recognize the fact that an aquifer to provide a given ground-water supply requires a surface area drainage area of at least equal to and sometime greater than that which would be required for the same supply derived from natural surface runoff. Lacking this, an aquifer may provide abundant supply for a time bur gradually fails through lack of replenishment. While there are areas from which limited ground-water supplies can be obtained in the vicinity of Rochester, There is apparently none having the requisite underground storage capacity and surface intake area for so large a supply as is required. Even if ground-water of adequate quantity and suitable quality was obtainable, it would require pumping, whereas an adequate supply can be obtained from the Hemlock system without pumping.