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Water Testing Results
Summer 2009

Souhegan and Merrimack Rivers

George May's Latest Results Summary
Map Showing Test Locations

The Souhegan Watershed Association monitors the entire length of the Souhegan River and part of the Merrimack River for their aquatic health. Water samples are tested for pH, phosphorus, dissolved oxygen, temperature, and bacteria. Weather and streamflow information was also recorded when water samples are collected. Previous results, beginning in 1997, are available in reports that can be found in local libraries. Summaries and original results are available here.

This monitoring program is conducted by trained volunteers who believe in cleaner rivers. Financial support for the program comes from your memberships and donations. If you would like to help continue this monitoring effort, please contact George May at 883-3409 or "georgemay (at) comcast (dot) net" or send a check to SWA, PO Box 1474, Merrimack, NH 03054.

E-Coli Bacteria Counts

Our E-coli samples are prepared and counted by professionals at the local wastewater treatment plants in Greenville, Merrimack, Milford, and Nashua on a volunteer basis. The results are reported as the number of e-coli bacteria colonies observed under a microscope in 100 ml of water. Bacteria levels below 88 colonies per 100 ml. are considered safe for public swimming areas. Anything above that level may cause ear and eye infections, diarrhea or other gastrointestinal problems in a percentage of swimmers. These bacteria numbers being reported are good for only a few days. They do indicate the general health of the river water at a particular moment in time. The next measurement can be considerably different, depending on water level and temperature, and what has recently washed, flowed, or been poured into the river. The current E-coli results this year are available here.

Dissolved Oxygen Measurements

Dissolved oxygen is "breathed" by fish and insects that live in the water. Dissolved Oxygen requirements for different species vary greatly, even if only fish are considered. One of the more sensitive family groups is trout. The lower limit for them is about 5 mg/L (or 5 ppm). The maximum value attainable is called O2 saturation. The concentration of O2 equal to the saturation limit in water increases with DECREASING temperature and INCREASING pressure. At 1 atmosphere pressure and 20 degrees centigrade the concentration of saturated O2 is 9.1 mg/L, while at 1 atmosphere pressure and 25 degrees centigrade O2 is saturated at 8.2 mg/L. The rate of oxygen usage, the rate of oxygenization (turbulence) and the rate of mixing of the different strata of water all contribute to surface oxygen levels. The simplified conclusion is that adequate oxygen levels indicate a healthy, balanced river habitate.

Cumulative 2009 E-Coli Data

August 18th Report

Hot weather and lower river flows normally increase the E. coli bacteria levels during the summer. That’s what the water quality monitoring program showed this week on the Souhegan and Merrimack Rivers. And that’s what this test would indicate for all NH and MA rivers also.

The testing program sponsored by the Souhegan Watershed Association and the Merrimack River Local Advisory Committee looks for a level of less than 88 colonies of E. coli that would indicate the safest water for swimming. Levels higher than 88 would result in the closing of any public swimming areas until the count came back down. There are no public swimming areas on the Souhegan or Merrimack so the state uses a level of 126 colonies for non-public “swimming holes.” Higher levels may cause ear and eye infections and intestinal distress in some swimmers.

The popular “swimming holes” on the Souhegan River came with mixed results. The Horseshoe in Wilton tested at 36, a very safe level. The Boston Post Road Canoeport in Amherst tested at 93, slightly over the 88 level; however, this test was done with an inadequate amount of water and may not be correct. The Turkey Hill Bridge area in Merrimack was 117, still less than the 126 safe level.

Generally the steeper, rocky headwaters of the Souhegan tested very good until the river reached the area of The Oval in Milford. As the river meandered through the slower, sandy sections through Milford, Amherst, and Merrimack, the bacteria levels stayed well above the 88 safe-swimming level. The highest number, 291, was recorded downstream of the Amherst Country Club and Beaver Brook. “Everything here was expected,” according to George May, SWA president.

The Merrimack River usually tests very good for bacteria levels. All of the sites between Manchester and Merrimack were well below the 88 level, but they increased slightly through Nashua. The site just upstream of the Tyngsborough Bridge had the highest of any site tested on both rivers. This summer several tests on the Merrimack have tested much higher than expected.

The flow on both rivers has dropped substantially since it stopped raining and hot weather prevailed. But both rivers are still much higher than normal for this time of year. The Merrimack should be flowing around 1510 cubic feet per second; it was double that at 3660 at the time of the test. The Souhegan should be 46 cfs; it was triple that at 138.

High dissolved oxygen levels are important for the fish and other organisms in the river and provide an important picture of a river’s health. Generally dissolved oxygen levels drop during the summer months. Until now DO levels have been excellent in both rivers. Now they are starting to drop. One site on the Souhegan upstream of Waterloom Pond in New Ipswich was measured at 5.32, the lowest seen this season. DO levels below 5 are unhealthy. All other sites were in the very healthy 7-8 range.

Monitors saw all kinds of wildlife: beavers, ducks, blue and green herons, geese, lots of frogs, and swimmers and boaters while they were out collecting samples early Tuesday morning. They noted Joe Pye Weed and water lilies in bloom. Unfortunately, they also noted trash at a few sites and an oil slick at Thorntons Ferry in Merrimack.

The tests for E. coli and dissolved oxygen were performed by certified lab technicians at the Merrimack, Milford, and Nashua Wastewater Treatment Facilities. Sample collections, water and air temperatures, and observations are done by the volunteers at their sites. This is the thirteenth year that this local volunteer program has been underway.

August 4th Report

E. coli counts on the Souhegan and Merrimack Rivers continued to be generally good again this week when tested by the volunteers in the SWA monitoring program sponsored by the Souhegan Watershed Association and the Merrimack River Local Advisory Committee. There were a few exceptions on both rivers that showed slightly elevated counts, but generally both rivers looked healthy.

Bacteria levels in the headwaters of the Souhegan looked as good as they’ve ever been. It was not until the river reached the Oval in Milford that levels jumped slightly higher than the 88 count that is the ‘gold standard’ for swimming. The count measured at the Swing Bridge was 99. The level stayed above the 88 mark through Amherst and Merrimack. This is usual for this slower moving, meandering stretch of the river. Two swimming holes on the river tested above the 88 level. The Amherst Canoeport on Boston Post Road was 105. The area upstream of the Turkey Hill Bridge in Merrimack was 124. Both of these levels are acceptable for non-public swimming areas.

The Merrimack River below Nashua seems to have recovered from a couple of high readings two weeks ago. Readings this week were all below the acceptable 88 level. However, the pollution seems to have moved above Nashua this week. The test taken just upstream of where the Souhegan comes in tested at 488, quite high for the Merrimack. The levels dropped at Thorntons Ferry and were acceptable again by the time the river reached the Greeley Park boat ramp. According to George May, president of the SWA, the pollution increased somewhere between Depot Street and the Souhegan River. There are no brooks coming into the river between those two sites,”

A map showing the location of the sites tested and the E. coli bacteria levels all summer on both rivers is available at the SWA website at and at the Nashua Telegraph’s website at

The flow on both rivers, and all rivers in the area, is still very high. A number of boaters and swimmers have gotten in trouble because of the high flows this summer. Both the Souhegan and Merrimack were about ten times as high as they should be at this time of year. Both were double the flow from two weeks ago. At the end of last week the flow on the Merrimack was 20,000 cubic feet per second. At this time of year, the flow should be around 2,000. “Kayakers and experienced boaters have enjoyed an exceptional summer; inexperienced boaters have gotten in lots of trouble and should be careful out there,” according to May, an experienced whitewater kayaker.

All of the tests for E. coli and dissolved oxygen are performed by certified lab technicians at the Merrimack, Milford, and Nashua Wastewater Treatment Facilities. Sample collections, water and air temperatures, and observations are done by the volunteers at their sites. This is the thirteenth year that this local volunteer program has been underway.

July 21st Report

E. coli counts on the Merrimack and Souhegan Rivers were at generally acceptable levels when tested on Tuesday morning by the all-volunteer water quality monitoring program sponsored by the Souhegan Watershed Association and the Merrimack River Local Advisory Committee. But there were a couple of unexpected exceptions.

Testing on the Merrimack River produced very high readings near the Sagamore Bridge and again near the Tyngsborough Bridge. This was unexpected because the Merrimack River almost always tests very clean as far as the E. coli bacteria counts go. The counts do rise dramatically to the numbers seen here after very heavy rainstorms that cause the Nashua Wastewater Treatment Facility to be overwhelmed. When that happens, the water from the combined street stormdrains and the sewers is released directly into the river at up to eight different sites in Nashua. This could be the cause of the unexpected counts. But the weather was completely clear for the three days prior to the tests and for the normal lifespan of the bacteria. There was a thunderstorm four days prior to the testing but the treatment plant should have been able to handle that event without a CSO discharge.

The flow on both rivers was high for this time of year as a result of all the recent rain. Flow on the Merrimack was double what it should be at this time of year. Flow was 4190 cubic feet per second. This is higher than the historic average of 1780 but still low enough that all of the flow can be used by the generators at Amoskeag in Manchester to produce electricity. The generators there can use up to 4500CFS before spilling the excess over the dam.

But in spite of the higher flow, the Merrimack River looks lower to anyone passing over the bridges in Nashua and downstream. This is because the dam in Lowell has lowered the water to work on the dam. This has been a problem for several years now according to a spokesman for the Merrimack River Local Advisory Committee. “They keep lowering the water in the pond that extends back upstream of Nashua way too often,” according the LAC. “This causes problems with boaters having to dodge rocks and causes serious erosion along the banks.” The LAC has been monitoring erosion that could endanger buildings at Thoreau’s Landing at the end of Lock Street in Nashua.

E. coli counts on the Souhegan River were generally good. Although a number exceeded the 88 count level that is the standard for public swimming areas and the gold standard the testing program looks for, they all fell within the acceptable level for this program. Here also there was an unexpected test. The Horseshoe in Wilton usually tests better than other sites on the river – almost always below the 88 threshold for public swimming. But this time the count came in at 135 and was the highest count on the entire river. The Horseshoe is a popular swimming hole on the Souhegan. At this level swimmers could experience ear and eye infections and gastrointestinal problems.

There were very few tests done on the upper section of the Merrimack River because of a lack of volunteers. Anyone interested in helping with the biweekly testing can contact or for more information. The tests take about a half hour every other Tuesday morning. The next tests will be done on August 4th.

July 7th Report

Good weather this past weekend was also good news for local rivers. Both the Merrimack River between Manchester and Tyngsborough and the entire length of the Souhegan River had very low E. coli counts when tested by volunteers with the Souhegan Watershed Association and Lower Merrimack Local Advisory Committee. The absence of heavy rain from Saturday through Monday allowed the rivers to flush out some of the moderately high bacteria colonies seen at the prior test two weeks ago. Unfortunately the volunteer monitors weren’t so lucky as light showers were getting them soggy during the early morning Tuesday test time.

Monitors on both rivers noted the high flow caused by the incessant heavy rains for the last month. The Souhegan was flowing at 600 cubic feet per second, ten times the expected historical flow for this date. This is a little lower than the flow rate two weeks ago but much higher percentage wise compared to its expected historic flow.

The Merrimack River was flowing at 12,000 cubic feet per second, much higher than two weeks ago. The Merrimack has been flowing at very high levels for this time of year. Last weekend the flow rate was over 20,000 CFS and caused a scheduled canoe trip sponsored by the Local Advisory Committee to move the trip to a safer river. To give a comparison of how much water that is, more water was flowing down the Merrimack every second that the average household uses in a year.

The E. coli count never reached the 88 count level that the program uses as its gold standard on either of the two rivers except for one site in Amherst that is just below the incoming flow from Beaver Brook. That site’s count was 96, still within the standards for safe swimming, boating, and fishing.

This test was a rare treat for this time of year. Very few tests in the last dozen years that SWA has been monitoring the rivers have not had any E. coli counts exceeding the 88 limit. So far the three good tests taken this year may indicate the increasing health of the rivers. The Merrimack especially has shown real improvement over the course of the last dozen years of monitoring.

SWA is still looking for a few volunteers to monitor sites along the Merrimack. Those indicating NA, not available, need monitors. Tests are done every other Tuesday early morning and take about half an hour. Prospects may contact karenmattor (at) or georgemay (at) for information. The next tests will be done on July 21.

June 23rd Report

E. coli bacteria numbers increased on the Souhegan, Nashua, and Merrimack Rivers during the most recent tests performed by the local watershed associations. Generally E. coli bacteria counts increase during rain events and this proved to be the case this week. Constant rains have been washing pollution off river banks and from storm drains causing the E. coli counts to increase.

The Merrimack River tested quite good. There were only a limited number of sites tested between Manchester and Tyngsborough, but all of those were below the 88 count considered acceptable for public swimming. Tests were performed in Manchester, in Nashua, and in Tyngsborough so it may be inferred that the missed sites in between were also acceptable. This has generally been true for the Merrimack River for the last several years of testing. Only after extremely heavy rains that overwhelm the Nashua Wastewater Treatment Plant and causes raw sewage to flow into the river do the readings get high enough to be concerned about.

The Souhegan River had only two of the 16 sites tested fall under the acceptable 88 count. Both of these were in the upper half of the river between New Ipswich and Wilton. The upper half of the river is more like a mountain stream with boulders, rapids, and rushing current. The lower half from Milford to Merrimack crosses flatter terrain and meanders more slowly through a sandy bottom and eroding banks. This lower half usually has higher E. coli counts as it did this time. The average count for this section of the river was 225, well above the level for safe swimming. Generally the E. coli counts increased as the river moved along. A count of 345 was measured at the Amherst Canoeport on Boston Post Road. This spot is a popular swimming hole during the summer. At Turkey Hill Bridge in Merrimack, another swimming spot, the count was a lofty 276. The other popular swimming spot on the Souhegan is The Horseshoe in Wilton. This is in the upper half of the river and the bacteria count there was an acceptable 68. Counts higher than 88 may cause intestinal problems and ear and eye infections in some swimmers.

The Nashua River Watershed Association tests the entire length of the Nashua and many of its tributaries. Seven sites in the Nashua area are reported here and they contain two of the highest counts seen during this testing. Walker Brook in Mason was over 500 and the canoe launch in Pepperell was over 1000.

All of the tests are performed by watershed volunteers. The lack of reporting along the Merrimack points out the need for more helpers. Anyone willing and able to spend an early morning half hour at the river every other Tuesday can contact coordinator Karen Mattor at or George May at We provide all the equipment and training.

All of the lab tests are performed by the Milford, Merrimack, Nashua Wastewater Treatment Facilities using state of the art equipment. Volunteer monitors perform the field tests and collect the samples to transport to the labs.

June 9th Report

A drizzly, light rain greeted monitors on the Souhegan and Merrimack Rivers for the first day of the summer water quality monitoring program. Tuesday began the 13th consecutive year that the Souhegan Watershed Association and the Merrimack River Local Advisory Committee have been monitoring both rivers. All of the monitors are volunteers that spend about a half hour every second Tuesday morning collecting samples.

Although the weather may not have been perfect for the monitors, it is better for the sampling data. Rain washes pollution off the river banks and down storm drains and gives a better indication of possible problems with the water quality.

The data over the past 12 years indicates a cleaning up of both rivers. Generally the water quality on both rivers is good - at least for the bacteria testing. There are other pollutants in both rivers that the program does not test for however. The Souhegan has shown fairly high counts of lead and phosphorus when tested in the past. The Merrimack is still recovering from decades of dumping by factories along the shore. Creosote is still leaking into the river upstream of Greeley Park.

E. coli bacteria counts showed a fairly clean Souhegan. Counts were slightly elevated as the river passed through the downtowns of Wilton and Milford, but they were still relatively low and generally acceptable. Counts higher than 88 are considered unsafe for public swimming spots and these areas were slightly higher than that, but they met the water quality standards for these rivers.

The Merrimack River between Manchester and Tyngsborough came in with very low E. coli bacteria counts. The highest number here was lower than the best site tested on the Souhegan. This has been true for several years now. As far as bacteria go, the Merrimack is almost always clean except after heavy rainstorms. There were reports of a soapy discharge near the confluence with Salmon Brook, but this did not show up in the tests performed by this program.

Testing for E. coli and dissolved oxygen are performed in the labs at the Milford, Nashua, and Merrimack wastewater treatment plants. Temperatures and observations are done by the monitors in the field.

A number of the sites were not tested due to lack of volunteers to monitor the sites. If anyone would like to adopt one of these sites, contact or The next test will be done on June 23.

E coli counts for NH portions of the Nashua River are also shown here. The Nashua River Watershed Association monitors the entire length of the river once a month during the summer.

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