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Carbon Cycling in Northern PeatlandsGeophysical Monograph Series 184Copyright 2009 by the American Geophysical Union.10.1029/2008GM000825

The Influence of Permeable Mineral Lenses on Peatland Hydrology

A. S. Reeve and Z. D. Tyczka

Department of Earth Sciences, University of Maine, Orono, Maine, USA

X. Comas

Department of Geosciences, Florida Atlantic University, Boca Raton, Florida, USA

L. D. Slater

Department of Earth and Environmental Sciences, Rutgers, State University of New Jersey, Newark, New Jersey, USA

Cross-sectional computer models were created that incorporated different high permeability zones to explore the potential role of eskers and similar geologic units associated with peatlands on the hydrology of these systems. These computer simulations indicate that small isolated lenses of high permeability material will locally distort the flow field, shifting flow patterns and creating new discharge and/or recharge zones. The simulation with the greatest hydraulic connectivity between the peat dome and peatland lagg displays widespread and continuous-with-depth downward flow beneath the bog dome, consistent with field observations. These simulations were compared with field data collected from a Maine (USA) peatland, and they suggest an esker identified within this peatland distorts the hydraulic gradients and resulting flow patterns within the peat. Similar subsurface features may be important in other peatland systems.

1. INTRODUCTION

Hydrology exerts an important influence on the vegeta-tion patterns and the geomorphology of peatland systems by controlling the saturation state and the transport of nutrients within the peat column. The movement of water within the peat is controlled by the water table position and the perme-ability field within the peat and geologic materials under-lying the peat. Several conceptual models, summarized by

Reeve et al. [2000, 2001b], have been proposed to describe the hydrology and water chemistry of peatland systems.

Reeve et al. [2000] described the impact of mineral sedi-ment permeability underlying peat on groundwater flow pat-terns, particularly vertical flow, within the peat column. They noted a reduction in the vertical penetration of groundwater flow cells beneath a bog dome when the mineral sediment permeability underlying peat deposits decreased. These re-sults have important implications for identifying where ver-tical flow is negligible in peatland systems and under what conditions vertical flow will be an important component of groundwater flow within peat deposits. In this paper, we ex-pand on this work and assess the impact of isolated lenses of sand and/or gravel underlying peatland systems on flow patterns within the peat. We hypothesize that the assumption of homogeneous stratigraphy beneath peat deposits, as has

290 INFLUENCE OF PERMEABLE MINERAL LENSES ON PEATLAND HYDROLOGY

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been made in many studies [Ingram, 1982; Winston, 1994; Reeve et al., 2000; Glaser et al., 2004], may oversimplify flow patterns within peat and obscure important hydrogeo-logic characteristics of peatlands.

Many researchers have explored the influence of heteroge-neity within peat [Beckwith et al., 2003] or within geologic materials [Freeze and Witherspoon, 1967; Winter, 1976], but we are unaware of any systematic study that explores the in-fluence of permeable lenses of mineral sediments on the flow patterns within peatlands. The importance of subsurface con-ditions on peatland hydrology has been widely recognized. The importance of low permeability units beneath a New-foundland peat complex was assessed using cross-sectional computer models. That analysis identified the permeability of peat at the margin of the peatlands as particularly impor-tant in regulating water levels within a peatland [Lapen et al., 2005]. The structure and stratigraphy of geologic materi-als may also direct groundwater discharge into a peatland [Glaser et al., 1997; Siegel, 1992; Boldt, 1986], supplying solute-rich water to the peatland system. Geologic features may also hydraulically isolate peatland systems from nearby surface hydrology features [Bradley, 2002] or provide path-ways for nutrient loading [Drexler and Bedford, 2002]. Co­mas et al. [2004, 2005a] used ground penetrating radar to image a subsurface feature beneath the central unit of Cari-

bou Bog, Maine (USA) interpreted as an esker deposit. They speculate that the presence of this esker influenced the gen-esis of pools now present in this peatland.

2. METHODOLOGY

In this paper, we present results from several computer models that incorporate different permeable units beneath a peat deposit. The results from computer simulations are compared with field data collected from Caribou Bog, a peatland system associated with an esker deposit.

2.1. Computer Simulations

Computer simulations were run using FiPy [Wheeler et al., 2007], a modeling package developed at the National Institute of Standards and Technology (USA). This software package is a general partial differential equation solver based on the finite-volume method. Each model simulates flow in a cross-section through an idealized peat basin. The cross-sectional model is 3000 m long with a maximum thickness of 25 m. This area represents a peat basin with upland hills on either end of the modeled area.

All sides of the model domain, except the top, were as-signed no-flow boundary conditions (Figure 1). Constant

Figure 1. Configuration of computer models, including the grid used to simulate groundwater flow. The model domain is 23-m thick (maximum) by 3000-m wide. The areas for the peat and sediment lenses indicated in the upper figure were used to select cells to assign different hydraulic parameters by determining if a cell’s center was within the different lay-ers or lenses.

REEVE ET AL. 291

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head boundaries were assigned along 100-m strips at the top of the peatland where the peat intersects the mineral soil (head equal to surface elevation), to simulate the stand-ing water in a lagg. Recharge (R) assigned to the top cells in each simulation varied with the material present at the surface, with higher recharge rates assigned to peat (15.7 cm a−1) and lower recharge rates assigned to the lower per-meability glacial sediments (7.9 cm a−1). Evapotranspiration rates (ET) varied with water table depth, changing linearly from a maximum value (3.2 cm a−1) to 0 as the water table drops from the surface to a meter below the surface. The ET term is intended to simulate the removal of water through both surface runoff and ET, preventing the water table from rising far above the land surface.

Hydraulic conductivities assigned to the geologic units in the computer model are based on values measured at Caribou Bog. Peat deposits within the basin are subdivided into an upper permeable layer (1-m thick) and a lower less permeable layer (remaining thickness of peat). The hydrau-lic conductivity assigned to the upper peat layer decreased exponentially with depth from 1 × 10−3 to 1 × 10−4 m s−1. The deep peat was assigned a hydraulic conductivity of 5 × 10−6 m s−1 based on measurements in Caribou Bog [Stevens, 2006]. Glacial sediments in the simulations were assigned

conductivities of 1 × 10−7 m s−1, based on measurements within till that outcrops adjacent to Caribou Bog at a land-fill site [Cole, 1992]. Five areas were predefined within the grid to represent heterogeneity within the geological materi-als beneath the peat. These sediment lenses were assigned a hydraulic conductivity of 1 × 10−4, representing sand and/or gravel lenses within the till.

Three steady state computer models of a generalized peat-land system were constructed to evaluate the potential impact of isolated permeable deposits on long-term flow patterns. Simulations were run assuming a ratio of horizontal (Kxx) to vertical (Kzz) hydraulic conductivity of 1 (Figure 2) and 10 (Figure 3). For each set of simulation scenarios, additional permeable features in the glacial sediment were intended to enhance the degree of lateral hydrogeologic communication within the subsurface. All models assume Darcy’s law ad-equately describes the flow of water through the peat [He­mond and Goldman, 1985]. A diffusion equation was used within FiPy to calculate the hydraulic head (h) distribution in the model domain:

0 =¶¶ x

Kxx¶ h¶ x

+¶¶ z

Kzz¶ h¶ z

+R − ET. (1)

Figure 2. Results from computer simulations with isotropic hydraulic conductivity incorporating: (a) no heterogeneity in the mineral sediments, (b) two high hydraulic conductivity lenses in the glacial sediment, and (c) five high hydraulic conductivity lenses in the glacial sediment. For each of these scenarios pictured, the flow domain (3000-m long and 24-m high) is displayed with clipped left and right edges below an image focusing on the left portion of the peatland. The gray scale of the image is related to the hydraulic conductivity of the material, with lighter colors indicating higher hydraulic conductivity. Streamlines are indicated with thick black lines, and equipotentials are indicated by thin dark gray lines at a contour interval of 5 cm (equipotentials are not shown in the mineral sediment). Vectors indicating the magnitude and direction of flow are included in the detailed images.

292 INFLUENCE OF PERMEABLE MINERAL LENSES ON PEATLAND HYDROLOGY

fig3

To account for desaturation of cells within the model, the methodology described by Doherty [2001] was used. This method involves smoothly decreasing the horizontal hy-draulic conductivity until it reaches a negligible value and smoothly increasing the vertical hydraulic conductivity to some large value as the water level in a cell drops below the cell bottom. This results in negligible amount of horizon-tal flow in “dry” cells, while increasing the vertical flow to allow hydraulic source and sink terms assigned to the top cells to communicate with the active “wet” cells within the model.

The described simulation approach has a number of shortcomings. Cross-sectional simulations of peatland eco-systems assume there is no flow perpendicular to the cross section. As bog domes in peatlands produce radial flow pat-terns, this assumption is violated. Steady state simulations fail to account for variation in hydraulic stress controlled by seasonal and shorter term changes in precipitation and eva-potranspiration. These temporal changes in hydraulic stress produce important seasonal or event-driven changes in flow within peatland systems [Devito et al., 1997; Reeve et al., 2006] that are not simulated in steady state models. Within peatland systems, biogenic gas [Rosenberry et al., 2006;

Kellner et al., 2005; Romanowicz et al., 1995] and chang-ing water table position [Price and Schlotzhauer, 1999] may alter the hydraulic conductivity of peat over time by occluding pore space and changing the stress field within the easily deformed peat. Despite the false assumptions used in our computer models, they do allow the variation in mineral sediment permeability to be isolated and evaluated through numerical experiments.

2.2. Field Measurements

Hydraulic heads were measured along two transects (T1 and T2, Figure 4) across Caribou Bog in 1999 and 2000 to evaluate seasonal variability in hydraulic head and flow pat-terns. A short transect (T3) was installed in 2006 to evaluate the hydrologic influence of an esker deposit [Comas et al., 2004, 2005a] buried beneath the peat. Caribou Bog is a mul-tiunit peatland located in central Maine between the cities of Orono and Bangor and covers an area of about 2200 ha [Davis and Anderson, 1999] with peat thickness of up to 18 m (Figure 4). The central portion of Caribou Bog contains a raised bog surrounded by fen. A fen water track, a peat land-form with ribbed pools, is located north of the central unit’s

Figure 3. Results from computer simulations with a tenfold horizontal to vertical hydraulic conductivity anisotropy incorporating: (a) no heterogeneity in the mineral sediments, (b) two high hydraulic conductivity lenses in the glacial sediment, and (c) five high hydraulic conductivity lenses in the glacial sediment. For each of these scenarios pictured, the flow domain (3000-m long and 24-m high) is displayed with clipped left and right edges below an image focusing on the left portion of the peatland. The gray scale of the image is related to the hydraulic conductivity of the material, with lighter colors indicating higher hydraulic conductivity. Streamlines are indicated with thick black lines and equipotentials are indicated by thin dark gray lines at a contour interval of 5 cm (equipotentials are not shown in the mineral sediment). Vectors indicating the magnitude and direction of flow are included in the detailed images.

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fig4

main raised bog unit [Davis and Anderson, 1999]. An intri-cate complex of narrow pools up to 200-m long occurs on the bog dome near the intersection of transects 1 and 2.

Monitoring wells installed along these transects were made from 1.91- or 2.54-cm nominal diameter PVC pipe. These flush-threaded pipes were installed to depths ranging from 1 to 18 m below the peat surface by manually pushing the well into the peat. Monitoring wells were installed in clusters of three or more wells fitted with 30-cm long machine slotted screens. Before installing wells, the mineral soil depth was estimated using either a 2.54-cm diameter piston corer or by probing the peat with a 2.54-cm diameter rod.

The top of a monitoring well at each well cluster was sur-veyed using an Ashtech dual frequency GPS. GPS data was postprocessed to differentially correct the data relative to a National Geodetic Survey reference station. Standard er-rors calculated by postprocessing software were less than 1 cm. Resurveying a monitoring well at one station yielded a difference of less than 1 cm, supporting the low calculated standard errors and the accuracy of dual frequency GPS. El-evation differences between wells within each cluster were measured periodically using a carpenter’s level to assess movement over time within each cluster, and little change was observed. Before measuring water levels, all wells were developed by pumping water from them until organic sedi-ment was no longer evident in the purged water. Water lev-els recovered in these wells following development in about 30 min to several hours. Water levels in monitoring wells

were measured using an electrical water-level indicator and converted to elevation relative to mean sea level using the surveying data.

3. RESULTS AND DISCUSSION

3.1. Computer Simulations

The baseline computer simulations (Figures 2a and 3a) are similar to previously published computer simulations of pristine bog-fen peatland complexes [Siegel and Glaser, 1987; Reeve et al., 2001a]. In these simulations, groundwa-ter flow cells develop under the peat dome, with downward flow near the center of this dome and groundwater discharge from the mineral sediments into the peat along the flanks of the peat dome. Little groundwater that has come in contact with the mineral sediments is advected into the upper por-tion of the peat column. There is little vertical flow through the peat column in the flat (fen) regions surrounding the bog dome, except near the peatland lagg where groundwater dis-charge occurs. The vector field in Figures 2a and 3a insets indicate that the majority of groundwater flux occurs in the upper, more permeable, peat, with much slower groundwa-ter flux rates in the deeper peat. The limited flow (indicated by the very short vectors) near the bog dome is due to de-saturation of the upper portion of the peat column near the crest of dome. Assigning tenfold anisotropy to the hydraulic conductivity of the different units decreases the amount of

Figure 4. Map of area near Caribou Bog, Maine. Darker colors indicate topographically lower areas. The location of three monitoring well transects are indicated by solid white lines. Circles mark the location of wells along transects 1 and 2.

294 INFLUENCE OF PERMEABLE MINERAL LENSES ON PEATLAND HYDROLOGY

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fig5

vertical flow while increasing the vertical hydraulic gradient. A second flow cell near the edge of the peat unit is present when hydraulic conductivity is isotropic, but is replaced by groundwater discharge originating from the upland under anisotropic conditions.

Introducing two lenses of permeable material near the bog dome (Figures 2b and 3b) distorts the flow field, increasing downflow through the peat but reducing the flux of water from the peat into the underlying till. These results are clearly dependent on the location of the high permeability field within the mineral sediment. Under anisotropic conditions, this distortion is less pronounced but still present. The addi-tion of high permeability units near the bog dome decreases the hydraulic head and lowers the water table position within the peat. In the isotropic case, the flux vectors in the peat have upward and downward vertical flow components on the left and right sides of each permeable lens, respectively. In the anisotropic scenario, increase in vertical flow is less pro-nounced, whereas vertical hydraulic gradients, illustrated by changes in equipotentials, are more apparent.

Increasing the number of permeable lenses from two to five (Figures 2c and 3c), extending them across the bottom of the peatland, results in cyclical shifts in vertical flow

across the peatland. Once again, shifts in the streamlines are more pronounced in the anisotropic scenario, and shifts in the equipotentials are more pronounced in the scenario with tenfold horizontal to vertical anisotropy in permeabil-ity. While these simulations alter the permeability within the mineral sediments, a similar insertion of high-permeability lenses in the peat will produce vertically converging and di-verging flow on the hydraulically upgradient and downgra-dient side, respectively. These results suggest recharge and discharge function of peatlands can be influenced by subsur-face shifts in permeability. We speculate that these shifts in vertical flow will produce variation in solute concentrations over short spatial scales, influencing vegetation communi-ties and geochemical processes within the peat.

3.2. Comparison to Field Data

Computer simulations are compared with hydrologic data collected in the central portion of Caribou Bog, a peatland that overlies isolated sandy deposits interpreted by Comas et al. [2005a] to be esker beads that roughly follow transect 1. Seasonal changes in water levels measured in the central por-

Figure 5. Cross-section along transect 2 with southeastern end on right side. Hydraulic head measurements (meters) collected in Au-gust 1999 are indicated next to well screen locations (boxes). Water table elevations are indicated above each well cluster. Equipoten-tials (dashed lines) are plotted at a one meter contour interval and inferred flow directions are indicated by solid lines with arrows. Gray shading indicates the location of mineral sediments underly-ing the peat.

Figure 6. Cross-section along transect 2 with southeastern end on right side. Hydraulic head measurements (meters) collected in April 2000 are indicated next to well screen locations (boxes). Water table elevations are indicated above each well cluster. Equi-potentials (dashed lines) are plotted at a 1-m contour interval, and inferred flow directions are indicated by solid lines with arrows. Gray shading indicates the location of mineral sediments underly-ing the peat.

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tion of Caribou Bog mimic the hydrographs for water bodies in temperate regions, with decreasing water levels through the late spring and summer and a recovery in water levels in the late fall and winter. Rapid increases in water level, sustained over weeks, are associated with snow melt in the spring (data not shown). Hydraulic head measurements in-dicate that the water table rises about 1.3 m from the lagg to the bog dome throughout the growing season, and these water elevations decrease by about 20 cm from late spring to late summer (Figures 5–7).

Hydraulic gradients indicate water generally moves radi-ally away from and downward within the peat dome. These flow patterns are driven by the water table mound beneath

Figure 8. Cross-section along transect 3. Mineral sediment locations are based on GPR profiles collected in this area [Comas et al., 2004]. Hydraulic head measurements (meters) collected in July 2006 are indicated next to well screen locations (boxes). Water table elevations are indicated above each well cluster. Equipotentials (dashed lines) are plotted at a 1-m contour interval, and inferred flow directions are indicated by solid lines with arrows.

the peat dome as indicated by the groundwater flow simula-tions. The persistent downward hydraulic gradients in the deeper portions of this peatland, and the shifts from verti-cally divergent to convergent hydraulic gradients along transects, (Figure 7) suggest additional factors are influenc-ing the hydraulic head in this peatland. Many researchers have invoked biogenic gas as the cause for anomalous hy-draulic head measurements in peat deposits, and overpres-suring observed in the central portion may be explained by the formation of gas. While gas production is clearly occur-ring within Caribou Bog [Comas et al., 2005b, 2007], com-puter simulations in this paper coupled with the observation of esker-like sediment mounds beneath the peat [Comas et

Figure 7. Cross-section along transect 1 with southern end on right side. Hydraulic head measurements (meters) col-lected in April 2000 are indicated next to well screen locations (boxes). Water table elevations are indicated above each well cluster. Equipotentials (dashed lines) are plotted at a 1-m contour interval, and inferred flow directions are indicated by solid lines with arrows. Gray shading indicates the location of mineral sediments underlying the peat.

296 INFLUENCE OF PERMEABLE MINERAL LENSES ON PEATLAND HYDROLOGY

al., 2004, 2005a], suggest an alternative explanation. Small local shifts in vertical flow may be controlled by high perme-ability zones in the mineral sediment and, perhaps, within the peat. Monitoring well transects that cross the esker ridge (Figures 5–6) show there are downward hydraulic gradients in the peat that overlies the esker. Hydraulic head data col-lected along a more detailed transect (Figure 8) support this interpretation. This pattern could be envisioned by travers-ing a simulated high-permeability unit on its hydraulically upgradient end perpendicular to the two-dimensional simu-lation. Groundwater flow driven by these hydraulic gradi-ents will alter the geochemistry within the peat, potentially creating zones favorable for (or inhibiting) microbial activ-ity and associated gas production.

4. CONCLUSION

Computer simulations of an idealized peat basin indicate that small isolated and permeable deposits will alter the flow patterns within a peatland, producing isolated recharge and discharge zones on the hydraulically up- and downgradient ends of these sediment lenses, respectively. Hydrologic data collected from a Maine (USA) peatland contain similar pat-terns, with vertical hydraulic gradients locally shifting from upward to downward. Downward hydraulic gradients were persistently measured over an esker-like feature identified by Comas et al. [2005a] beneath the peat. Computer simula-tions provide one possible explanation for shifts in vertical hydraulic gradients across peatlands, with these variations controlled by changes in the permeability of subsurface fea-tures beneath (and within) the peat. Identifying these features and interpreting their influence on the peatland hydrology will improve the broader understanding of these systems and may provide insight on vegetation patterning, solute fluxes, and gas production within peatland systems.

Computer simulation at the scale presented in this paper is a useful tool for evaluating the hydrology of peatland systems. These models provide an experimental framework to evalu-ate the importance of processes and the parameters control-ling those processes. Complex relationships that are difficult to isolate in laboratory or field studies, such as feedback be-tween biogenic gas production or carbon accumulation and hydrology, can be isolated in computer simulations.

Future computer simulation of peatland systems should attempt to link hydrologic, geochemical and biologic pro-cesses to better understand: (1) how these factors influence one another, (2) how peatlands will respond in various cli-mate change scenarios, and (3) the hydrologic role peat-lands play within the watersheds. Other peatland hydrology research amenable to computer simulation-based studies include the assessment of peatland heterogeneity and the

unique hydrologic properties of peat on the hydrology and solute transport within peatland systems.

Acknowledgments. J. Rhoades provided valuable field assistance and provided feedback on the computer modeling component of this project. The National Institute of Standards and Technology hosted Reeve for 1 month during a sabbatical. This work was sup-ported by the National Science Foundation (EAR-0510004) and the University of Maine.

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A. S. Reeve and Z. D. Tyczka, Department of Earth Sciences, University of Maine, Orono, ME 04469, USA. (asreeve@maine.edu)

L. D. Slater, Department of Earth and Environmental Sciences, Rutgers, State University of New Jersey, Newark, NJ 07102, USA.