E. coli Levels In Souhegan, Merrimack Rivers Returning To Normal

By George May

AMHERST, NH — At the last test two weeks ago, the rivers were slowly coming down from high flows when they were hit with a huge rainstorm just before the test. That rainstorm jumped the flows back up again and also greatly increased the E. coli counts on both rivers. The high flow kept a couple monitors from being able to safely monitor their sites and the E. coli counts went off the charts.

Almost every site on the Souhegan was above the 406 standard that meant it did not meet the standards for Class B waters, not swimmable or fishable.

This week the rivers have recovered. The flows are just about normal for this time of year. Averages are determined by going back 70 years. The upper Souhegan, from New Ipswich to Milford, is below normal right now, but that stretch is steep and rocky with fewer tributaries to feed the main channel. And that forecasts what is in store for the lower stretch which now is normal and drifting lower.

The biggest news this week is that the E. coli counts have also returned to normal. Actually better than normal. Every site tested on both rivers came in below the critical 406 level that determines state standards for swimmable, fishable waters. Watson Park in Merrimack tested at 59, well below the 88 SWA standards for excellent water quality. Some sites on the upper Souhegan and on the Merrimack had results in the single digits – better than excellent.

Here’s a chart of this season’s E. coli tests on the Souhegan. It starts on the left at the headwaters and flows to the right downstream to the mouth of the river. I have omitted the June 11 and July 23 tests because those high counts so skew the results that nothing else is clear. Here you can see that the river stays fairly clean until it gets to the Wilton/Milford town line at Site 201, then the E.coli count increases through Milford and Amherst, and then it cleans up some through Merrimack to the mouth of the river. This is its normal pattern.

Why is this? In Milford and especially in Amherst, the river slows down, meanders all over, becomes shallow, is very sandy, heats up, and has lots of tributaries and wetlands. Increasing E. coli here is normal and natural.

Dissolved oxygen levels are staying up in the very healthy range on both rivers. This is the best single measurement to determine the general health of the river. DOs normally decrease as the summer heats up and flows decrease. This summer, and most summers in the past, DO levels have remained very healthy. Interestingly, the rainstorms that bring the dreaded E. coli counts also bring desirable DO counts. One is bad for humans; the other good for fish.

We also measure pH levels on a few sites to get a general picture of river health. Again this summer pH levels are in the close to normal range, just slightly acidic. “Acid rain” is usually blamed for the acidity usually seen. This may be improving as more coal fired plants are being closed and less carbon dioxide is floating in from the west. Today’s readings look pretty good. Unfortunately we don’t have reliable data from our testing far enough in the past to make predictions but it seems as if things are getting better.

Generally this week both rivers are as good as could be asked for.

Thank you to Jane Hemmingsen for collecting and compiling all the data this week.

The next test will be on Aug. 20 and the results will be available the following day or two. This will be the final test for this season.

Here are the results for this week’s tests. The results for the Merrimack River from Manchester have not arrived yet and will be updated on the SWA website when received.

E-COLI MONITORING RESULTS FOR TESTS TAKEN ON AUG. 6, 2019

SOUHEGAN RIVER SITES: mpn/100mL

1. Billy Ward Pond, Ashburnham #1; #2 1; 5.2
2. SoR 333 Water Loom Pond, New Ipswich 4.1
3. SoR 320 Highbridge, New Ipswich 14.6
4. SoR 309 Above Greenville Mill Pond, Greenville 46.4
5. SoR 296 Downtown Greenville upstream of WWTP 28.8
6. SoR 291 Green Bridge off Rte 31 below Greenville WWTP 34.1
7. SoR 218 Below the Horseshoe, Wilton X
8. SoR 210 Downtown Wilton 41.4
9. SoR 201 Pine Valley Mill, West Milford 58.1
10. SoR 170 Behind Hayward Field, West Milford 214.2
11. SoR 155 Souhegan Valley Boys and Girls Club, Milford X
12. SoR 146 Swing Bridge, Milford X
13. SoR 133 Riverside Cemetery, Milford 172.5
14. SoR 130 Behind Lorden Plaza, Milford 80.5
15. SoR 122 Amherst Country Club 131.4
16. SoR 116 Amherst Conservation land Fairway Road, Amherst X
17. SoR 095 Boston Post Road Canoeport, Amherst 131.4
18. SoR 057 Indian Ledges, Merrimack 64.4
19. SoR 034 Turkey Hill Bridge, Merrimack 39.3
20. SoR 001 Watson Park, Merrimack 59.1

MERRIMACK RIVER SITES:

• Mer 600 Above Amoskeag Dam, Manchester X
• Mer 590 Arms Park, Manchester X
• Mer 580 Upstream of Piscataquog River, Manchester X
• Mer 570 Goffs Falls, Litchfield X
• Mer 560 Depot Street, Merrimack X
• Mer 550 Upstream of Souhegan River, Merrimack 10.7
• Mer 540 Thorntons Ferry, Merrimack 16.0
• Mer 530 Greeley Park, Nashua X
• Mer 520 Taylors Falls Bridge, Nashua 1
• Mer 510 Sagamore Bridge, Hudson 17.9
• Mer 500 Pheasant Lane Mall, Hudson X
• Mer 490 Upstream of Tyngsboro Bridge 3.1
• Mer 600 Above Amoskeag Dam, Manchester
• Mer 590 Arms Park, Manchester
• Mer 580 Upstream of Piscataquog River, Manchester
• Mer 570 Goffs Falls, Litchfield
• Mer 560 Depot Street, Merrimack
• Mer 550 Upstream of Souhegan River, Merrimack 8.32
• Mer 540 Thorntons Ferry, Merrimack 7.80
• Mer 530 Greeley Park, Nashua X
• Mer 520 Taylors Falls Bridge, Nashua 8.28
• Mer 510 Sagamore Bridge, Hudson 8.07
• Mer 500 Pheasant Lane Mall, Hudson X
• Mer 490 Upstream of Tyngsboro Bridge 7.49

Flow on Upper Souhegan was 15 CFS. Expected flow for that date is 40

Flow on Lower Souhegan was 48 CFS. Historic flow for that date is 45

Flow on Merrimack was 1250 CFS. Historic flow for that date is 1600

RAINFALL: Shower Sat; otherwise clear

Temperature: Souhegan 23C Merrimack 20C

DO MONITORING RESULTS FOR TESTS TAKEN ON AUG. 6, 2019

SOUHEGAN RIVER SITES: mg/L

1. Billy Ward Pond, Ashburnham #1 ; #2 6.64; 6.59
2. SoR 333 Water Loom Pond, New Ipswich 6.52
3. SoR 320 Highbridge, New Ipswich 8.12
4. SoR 309 Above Greenville Mill Pond, Greenville 8.40
5. SoR 296 Downtown Greenville upstream of WWTP 8.19
6. SoR 291 Green Bridge off Rte 31 below Greenville WWTP 8.59
7. SoR 218 Below the Horseshoe, Wilton X
8. SoR 210 Downtown Wilton 9.11
9. SoR 201 Pine Valley Mill, West Milford 8.88
10. SoR 170 Behind Hayward Field, West Milford 7.73
11. SoR 155 Souhegan Valley Boys and Girls Club, Milford X
12. SoR 146 Swing Bridge, Milford X
13. SoR 133 Riverside Cemetery, Milford 7.91
14. SoR 130 Behind Lorden Plaza, Milford 7.80
15. SoR 122 Amherst Country Club 7.84
16. SoR 116 Amherst Conservation land Fairway Road, Amherst X
17. SoR 095 Boston Post Road Canoeport, Amherst
18. SoR 057 Indian Ledges, Merrimack 8.67
19. SoR 034 Turkey Hill Bridge, Merrimack 7.44
20. SoR 001 Watson Park, Merrimack 8.96

MERRIMACK RIVER SITES:

SELECTED pH RESULTS FOR AUGUST 6, 2018

SOUHEGAN RIVER SITES: S.U.

1. Billy Ward Pond, Ashburnham #1 , #2
2. SoR 333 River Road above Water Loom Pond, New Ipswich
3. SoR 320 Highbridge, New Ipswich
4. SoR 309 Above Greenville Mill Pond, Greenville
5. SoR 296 Downtown Greenville upstream of WWTP
6. SoR 291 Green Bridge off Rte 31 below Greenville WWTP
7. SoR 218 Below the Horseshoe, Wilton
8. SoR 210 Downtown Wilton 6.96
9. SoR 201 Pine Valley Mill, West Milford 6.94
10. SoR 170 Behind Hayward Field, West Milford 6.70
11. SoR 155 Souhegan Valley Boys and Girls Club, Milford
12. SoR 146 Swing Bridge, Milford
13. SoR 133 Riverside Cemetery, Milford 6.71
14. SoR 130 Behind Lorden Plaza, Milford 6.84
15. SoR 122 Amherst Country Club 6.74
16. SoR 116 Amherst Conservation land Fairway Road, Amherst
17. SoR 095 Boston Post Road Canoeport, Amherst
18. SoR 057 Indian Ledges, Merrimack
19. SoR 034 Turkey Hill Bridge, Merrimack 6.90
20. SoR 001 Watson Park, Merrimack