WATER RESOURCES BULLETIN VOL. 13, NO. 2 AMERICAN WATER RESOURCES ASSOCIATION APRIL 1977
HYDROLOGY OF TWO SMALL WETLAND BASINS IN EASTERN MASSACHUSETTS'
Arnold L. O'Brien'
ABSTRACT: A hydrologic budget was prepared for two geologically different wetland basins in eastern Massachusetts for the 1971 water-year. Water table conditions prevailed at one wetland underlain by peat while an artesian system functioned at the other wetland which was underlain by muck. Hydrologic responses were generally similar at both wetlands, although each functioned differently in detail. Both wetlands exhibited high spring discharges and depressions of low flow. Ground water accounted for an estimated 93% of the total annual discharge from both wetlands; in late summer the peat deposit recharged the regional groundwater body. Evapotranspiration in the spring was retarded in probable consequence of the extreme wetness of the wetland soils. (KEY TERMS: wetlands; hydrologic budget; groundwater drainage; streamflow; evapotrans- pira tion .)
I NT RO DUC T I 0 N
Freshwater wetlands are those lands which are periodically flooded and which have ground water a t , or near the surface for a major part of the year. In addition, they are commonly floored with organic deposits such as peat and muck and are characterized b y a distinctive suite of plants. As a class o f land, wetlands have remained relatively undeveloped. Recent expansion and development in the New England area, however, threatens to destroy or seriously change many of the existing wetlands which, according t o one study cover 6%% of the total land surface of Massachusetts (Larson, 1973). In some localities where wetlands are numerous they may cover 50% of the area. Clearly wetlands in these areas are a major component of the regional hydrologic system.
The effects of wetland alteration on the hydrologic environment are difficult t o assess as there is little data on how the unaltered wetland responds in the natural hydrologic environment. Indeed certain types of wetlands may respond differently from other types necessitating hydrologic distinctions. The purpose of this research is t o investigate two geologically different freshwater wetlands as complete hydrologic systems.
'Paper No. 761 15 of the Water Resources Bulletin. Discussions are open until December 1, 1977. *Department of Earth Sciences, University of Lowell, Lowell, Massachusetts 01 854.
INSTRUMENTATION, CLIMATE, AND GENERAL DESCRIPTION
Two small wetland basins were selected in Lincoln, Massachusetts, a suburban town located 1 4 miles west northwest of Boston (figure I ) . The basins were instrumented and monitored during the I97 1 water-year t o record rainfall, snow accumulation, runoff, groundwater fluctuations, and soil moisture changes. On site precipitation measurements were supplemented by data from a permanent weather station a t L. G. Hanscom Field, Bedford, Massachusetts, located within three miles o f both wetlands. Continuously operating monitors were installed on some wells and stream gages were fitted with continuous monitors t o provide complete stream hydrographs. Both basins were checked for losses due to underflow and groundwater leakage. Measurements in 7-day units were applied t o the elements in the hydrologic equation: precipitation = surface runoff t baseflow t E T t leaks t change in soil moisture +change in groundwater storage. Specific yield (used t o calculate groundwater storage) was determined from the budget by convergent approximation. Measured soil moisture trends were matched t o calculated soil moistures t o yield the quantities used in the budget. Evapotranspiration was calculated as a residual from the hydrologic budget and a sinusoid was fitted t o these values by double integration. Quantities from the determined curve were entered in the budget.
The 1971 water-year was unusually dry; precipitation was nearly 20% less than the mean annual precipitation as judged by weather data from L. G. Hanscom Field, Bedford, Massachusetts. Monthly precipitation is compared to the period 1943-1965 in table 1 (data for the standard normal period, 1931 -1960 is not available for the Hanscom Field station). On a seasonal basis, spring and fall fell well below the norm, while summer
TABLE 1. Weather Data - L. G . Hanscom Field. Bedford. Massachusetts.
197 1 Water-Year Period 1943 - 1965 Mean Mean
Precipitation Temperature Precipitation Temperature Inches ( O F ) Inches ( O F )
October November Decein ber January February March April May June J uly August
2.39 3.36 5.28 1.54 3.76 2.1 2 1.98 4.27 3.21 3.34 I .97 1.90
54.5 44 .2 25.9 19.9 28.5 35.5 44.5 55.9 67.4 70.8 69.4 69.5
3.52 4.76 3.17 3.82 3.34 3.99 3.85 3.64 2.74 2.99 3.80 3 .50
52.5 41.7 28.9 25.8 27.3 35.8 46.4 57.2 66.4 71.7 69.3 61.5
35.1 2 48.8 43.71 48.7
Hydrology of Wetland Basins 327
0 I0 20 - miles
Figure 1 . Eastern Massachusetts.
precipitation was slightly below the mean. Temperature data indicates an unusually cold January followed by a slightly cooler than normal spring. Summer temperatures approximated the long-term mean while September (1971) was unusually warm as were the months of October and November (1970).
Route 2 Wetland The northernmost swamp, called the Route 2 wetland, is drained by Hobbs Brook
which flows easterly a distance of nearly one mile from the wetland and then turns south t o flow into Stony Brook (figure 2). The wetland covers 26.6% of the drainage basin being 0.329 square miles in extent and lying a t the mouth of a basin of 1.23 square miles. Here muck, approximately 3 feet thick, developed on glacio-fluvial deposits. Ground
0 .s 1 bow map by U.S.Geological Survey milos
Figure 2. Map of Wetland Basins.
Hydrology of Wetland Basins 3 29
water near the center of the wetland was observed to be under artesian pressure from October through July. The southern portion of the wetland is transected b y Route 2, a busy four-lane east-west highway that leads to Boston. Except for drainage ditches and the highway, there has been n o development within the wetland itself and the drainage basin is sparsely inhabited.
Cotiant Road Wetlarid
The Conant Road wetland lies two miles south of the Route 2 wetland and drains southward through an unnamed brook that empties into Stony Brook (figure 2). The wetland deposits cover an area of 0.279 square miles and lie a t the mouth of a drainage basin o f I . 12 square miles. This swamp covers 24.8% of the total drainage area, and is composed of peat averaging tcn feet thick which has accumulated in a basin bordered and bottomed by till.
Both wetlands are covered with stands of Red Maple broken occasionally with White Pine, Elm, and Tamarack. They drain southeasterly into the Charles River system which discharges into Boston Harbor. All drainage from both basins is derived from precipitation within the basin - no stream enters either basin. A history of past use, ditching, and recent reversion t o forest makes these wetlands quite typical of many of the small freshwater wetlands found in eastern Massachusetts.
GEOLOGY AND HYDROGEOLOGY Route 2 Wetland
The Route 2 wetland developed on a tongue of glacio-fluvial deposits that formed in a north-trending valley and were graded to standing water levels of glacial lake Sudbury (Koteff, 1964). There are no excavations in this deposit t o clearly reveal its structure, but 2% miles west, a t the Concord Town Landfill, there is a similar and probably contemporaneous deposit. Here deltaic deposits of silts and poorly sorted sands, gravels, and cobbles indicate rapid deposition near stagnant ice masses. Borings north of the Route 2 wetland indicate that a t this location the glacio-fluvial deposits may be more than 70 feet thick and occupy a northward sloping valley.
Mechanical analyses of sediments derived from a borehold and from near surface samples confirm that the glacio-fluvial deposits of the Route 2 Basin are poorly sorted. In fact, many of the layers are generally similar in grain size and sorting t o the till exposed throughout the area.
The hydrologic character of the deposit was investigated a t the mouth of the wetland where test borings were made. With the exception of the topsand layer, the permeability of the deposit averaged 3 gal/da/ft2 as determined with a variable head permeameter. Although the glacio-fluvial material is generally of low permeability, significant variation exists througbout the deposit and there are doubtless many lenses and layers of highly permeable sands.
The wetland deposits are generally shallow and are composed of an upper layer of black muck approximately one foot thick and a lower layer of brown silty peat. Test borings made with a harpoon borer (see figure 3) showed that the wetland deposits are
Route 2 Roadbed 7- 1 000 1,. b 2000 I,. a 0 1 I 1 1 1 1 0 -
Figure 3. Route 2 Swamp Sectional Profile.
generally three feet thick north of Route 2, but increase in thickness south of Route 2 where a maximum measured thickness of 9 feet was recorded.
Tests with a variable head permeameter showed that the muck which overlies the glacio-fluvial deposits was quite impermeable in the vertical direction but much more permeable in the horizontal direction (see table 2A). Low permeabilities for muck have been reported in the literature, but the observation of high horizontal permeability was previously unknown. Presumably it results from the effects of laterally spreading roots.
Conant Road Wetland
Unlike the Route 2 wetland, the Conant Road wetland is entirely surrounded by till. The organic deposit is peat averaging ten feet in depth with a maximum observed
Hydrology of Wetland Basins 33 1
thickness of 30 feet occurring along the eastern edge of the wetland (see figure 4). The upper foot of the wetland deposit is black and tends to be muck-like. Below this lies a mostly brown, but commonly red, aromatic peat. Near the bottom of the deposit is a grey-brown, compact, and fully decomposed peat. The peat is underlain by blue-grey silt
\ Limit of Boringr
C .- -20 f
t:igure 4 . Conant Road Swamp Sectional Profile.
which overlies the till and which probably represents a lake bot tom deposit. Several hillocks of till occur within the wetland and may stand as much as 5 feet above the level of the peat.
Permeameter measurements of the peat at the Conant Road Swamp indicate that the permeability decreases very rapidly with depth. High horizontal permeabilities were measured at the surface, but at greater depth horizontal permeabilities were quite similar t o vertical ones (see table 2B). Although high horizontal permeabilities have been reported in the literature, they were not observed here (with the exception of the surface layer), nor were they reported by Boelter (1965) who conducted an extensive investigation of peat permeability in Minnesota.
TABLE 2. Permeability Determinations on Peat and Muck.
Sample Depth Orientation Kpv* General Description
A . Route -3 Swamp
0 - 5 inches vertical .94 gpd/ft2 black muck 3% - 5 inches horizontal 110 gpd/ft2 black muck 3% ~ 5 inches horizontal 216 gpd/ft2 black muck 14 - 15% inches horizontal 2.8 gpd/ft2 silty brown peat 25 inches unoriented 102 gpd/ft2 fine sand
B. Conatit Road Swamp
0 - 5 inches vertical 28.6 gpd/ft2 black mucky peat 0 - 5 inches vertical 42.6 gpd/ft2 black mucky peat % - 2 inches horizontal 433 gpd/ft2 black mucky peat 4 - 5% inches horizontal 16 gpd/ft2 black mucky peat 16 - 21 inches vertical 2.1 gpd/ft2 fibrous peat 18% - 20 inches horizontal 1.6 gpd/ft2 fibrous peat
* Permeability in gallons per day per square foot determined on oriented samples in the field with a variable-head permeameter.
Two wells were installed at well site 6 t o measure groundwater fluctuations in the peat and in the underlying material. Well 6 was screened a t the bot tom of the peat deposit while well 6A was driven through silt deposits that underlie the peat into an oxidized sandy deposit. Hydrographs for the t w o wells are shown in figure 5.
The similarity in fluctuation and overall response suggests that the peat deposit is part of the same groundwater body that extends below the silt deposits. Water levels in well 6A were maintained at approximately 0.1 feet higher than well 6 indicating discharge of ground water into the peat from late November until August, For six weeks in the fall, water levels in well 6A fell below those of 6 and the swamp deposits recharged the regional groundwater body with an estimated 7 million gallons of water. Calculations of total areal recharge t o the basin for the week when maximum head differential occurred (week ending July 30) equaled 0.13 inches. This is approximately two orders of magnitude greater than the total runoff for that week (0.002 inches) suggesting that
Hydrology of Wctland kasins 333
1,'igure 5 . Conant Road Swamp Wcll Hydrographs.
recharge from the swamp deposits to the underlying aquifer could be significant during dry periods.
The hydrologic budget indicates that storage was depleated in both wetland basins a t the end of the water-year. Total groundwater deficit for the Route 2 basin was 0.46 inches or I .3% of the total precipitation. The Conant Road basin recorded a deficit of 1.2X inches or 3.6% of the total precipitation. The budget further indicates a basin wide soil moisture deficit of 4.23 inches. This is, however, a theoretical loss. Field evidence suggests the wetland soils had the same moisture content a t the end of the water year as a t the start.
Total evapotranspiration for the 1971 water-year is summarized below. PET and AET were calculated by the Thornthwaite method (Thornthwaite and Mather, 1957) as an aid in the interpretation of the data. The measurements suggest that the
measured ET Thomthwaite AET Thornthwaite PET Route 2 Basin 20.77 inches 22.81 inches 26.54 inches Conant Road Basin 23.37 inches 22.81 inches 26.54 inches
Conant Road Basin lost 12!4% more water to evapotranspiration than did the Route 2 Basin. Such a difference between two areas with nearly identical annual runoff and similar hydrologic response is difficult t o explain. In view of this, the data for total evaporative loss must be viewed skeptically.
The distribution of yearly evapotranspiration is shown in figures 6 and 7. Both wetland curves are depressed during the spring and early summer with respect to the theoretical curves, but the late summer and fall portion of the ET curves follow the theoretical curves (AET and PET) more closely. The occurrence of this phenomenon in both wetlands suggests the observatio...