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Pedology

Soil Morphology-Water Table Cumulative Duration Relationships in Southern New England

^a USDA-NRCS, Norwich, CT
^b Dep. of Natural Resources Science, Univ. of Rhode Island, Kingston, RI 02881

^* Corresponding author (mstolt{at}uri.edu)

	ABSTRACT

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Although redoximorphic features (RMFs) are commonly used in place of direct observation to estimate the depth of the seasonal high water table (SHWT), little is known about the relationships between RMFs and the duration of the water table. The objective of this study was to gain a better understanding of how the expression of RMFs relates to the depth and duration of the water table in moderately well-drained soils of southern New England. The 20 soils studied included Dystrudepts, Quartzipsamments, and Psammaquents; of these, 17 met the criteria for Oxyaquic (8) or Aquic (9) subgroup classification. Water table levels were monitored for 18 mo from February to July of the following year to compare the depth and cumulative duration of the water table (cumulative saturation) to RMFs within soil horizons. Average seasonal high water table level (ASHWT) was correlated (p < 0.05) to the depth of the first loamy horizon with >2% RMFs (r² = 0.65–0.72). Water table levels occurred within horizons with no RMFs for as much as 13% of the 18 mo study period. Mean cumulative saturation for soils with textures finer than loamy sand ranged from 3% for horizons with no RMFs to 36% for horizons with a depleted matrix. For horizons with textures of loamy sand and coarser, the mean cumulative saturation ranged from 8% for horizons with no RMFs to 45% for horizons with >2% depletions. (Percentage of cumulative saturation would have likely been less if we monitored for a full 2 yr, as the late summer and fall of the second year were not monitored.) Our results suggest that the abundance of RMFs increases as the percentage of time the water table is present within a horizon increases. Additionally, coarse-textured horizons are less expressive in regards to RMF abundance than are loamy-textured horizons with similar cumulative saturation. For land-use decisions, soil morphology should be used to provide both an estimate of the depths of a defined SHWT and cumulative saturation.

Abbreviations: ASHWT, average seasonal high water table • RMF, redoximorphic feature • SHWT, seasonal high water table

	INTRODUCTION

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GROUNDWATER is a dynamic system. The upper boundary of the saturated zone, commonly referred to as the water table, fluctuates in response to seasonal changes in evapotranspiration rates as well as in response to precipitation events (Daniels and Buol, 1992). Seasonal fluctuations in the water table are often more than 1.5 m (Simonson and Boersma, 1972; Thorp and Gamble, 1972; Fritton and Olson, 1972; Zobeck and Ritchie, 1984; Evans and Franzmeier, 1986; Cogger and Kennedy, 1991; James and Fenton, 1993; Khan and Fenton, 1994). These seasonal fluctuations can be highly variable between years due to variations in annual precipitation. Because of the dynamic nature of ground water and the associated water table, accurately predicting a long-term seasonal high water table (level) by direct measurement can be problematic.

In an effort to eliminate the problems associated with direct measurement of the SHWT, regulatory agencies have been adopting soil-based methods for making SHWT determinations (Vepraskas and Wilding, 1983; Zampella, 1994; RIDEM, 2000). Soil based methods rely on the interpretation of redoximorphic features to identify saturated conditions within the soil. Redoximorphic features form by the biogeochemical processes of reduction, translocation, and oxidation of Fe and Mn (Vepraskas, 1992). These processes are dependent on the presence of organic matter, microbes, and anaerobic conditions. The identifiable color patterns that result from these processes enable users to make predictions concerning the depth to the SHWT (Latshaw and Thompson, 1968; Daniels et al., 1971; Simonson and Boersma, 1972; Franzmeier et al., 1983; Cogger and Kennedy, 1991; James and Fenton, 1993; Veneman et al., 1998).

Although the use of RMFs to identify a depth to the SHWT is a proven approach, a number of studies have observed saturated soil horizons without RMFs (Daniels et al., 1973; Franzmeier et al., 1983; Pickering and Veneman, 1984; Evans and Franzmeier, 1986; Genthner et al., 1998; Calmon et al., 1998). These studies suggested that the presence of RMFs is not necessarily indicative of the depth of the water table, but how long the water table is at that depth or above (cumulative saturation). Therefore, if RMFs are used to estimate the SHWT, some measure is necessary that estimates the amount of time that the water table is at or above the SHWT depth. An estimate of the cumulative saturation would provide those making land use decisions dependent on SHWT indices some measure of the potential for the water table to be above that depth. For example, the amount of time the water table is present within a septic-tank drainfield treatment zone represents the time the treatment zone is potentially compromised. Thus, knowing the cumulative saturation provides a measure of the greater risk of ground water contamination. This study is a step toward establishing relationships between soil morphology and the depth and cumulative duration of the water table. Our objectives were to: (i) document the water table fluctuation patterns for representative, moderately well drained, glacial soils in southern New England and, (ii) examine the relationship between RMFs and the depth to, and cumulative duration of, the water table in these soils.

	MATERIALS AND METHODS

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Study Area
Block Island, located 20 km off the southern coast of Rhode Island (41°11' N, 71°35' E), is part of an end moraine formed as the Laurentide ice sheet retreated approximately 20 000 yr ago (Sirkin, 1996; Stone and Sirkin, 1996). A variety of glacial parent materials can be found on this 2600-ha island including loose till, dense till, and lacustrine and outwash sediments (Rector, 1981). In addition, loess and, immediately adjacent to open ocean, sand dunes cover portions of the glacial materials.

Site Selection and Sampling
A reconnaissance survey was performed using the Rhode Island Soil Survey (Rector, 1981) to locate study sites. Criteria used to choose study sites included: (i) soils were representative of the various types in Southern New England, (ii) soils were relatively undisturbed, (iii) soils were considered moderately well-drained based on morphology, (iv) sites could be continuously monitored over the duration of the study period, and (v) sites were reasonably easy to access and safe for monitoring equipment. A total of eight sites, formed in glaciofluvial, glaciolacustrine, loose till, dense till, loess, and eolian sand parent materials were selected for the study (Table 1). Each site was equipped with two or three wells located on backslope and footslope landscape positions. All wells were constructed and equipped with a maximum water table depth recording device (Morgan and Stolt, 2004) and periodically instrumented with data loggers (Infinities USA, Inc., Daytona Beach, FL, model 138). Coarse/fine stratifications at one site suggested that episaturated conditions may occur at a shallow depth. Therefore, this site was instrumented with a shallow well to establish if shallow episaturation was occurring. The wells were monitored on a biweekly basis from February 2001 to July 2002. During each site visit the current depth to the water table, as well as the maximum water table level since the previous visit, were recorded.

Data Analysis
To investigate relationships between soil morphology and cumulative saturation, soil horizons were placed into one of three groups. Two groups were based solely on textural class and rock-fragment content: 1) horizons with >70% sand (loamy sand and coarser) or had >35% rock fragments (coarse textured group), and 2) horizons with finer textures (loamy textured group). The third group was made up of soils with horizons formed exclusively in eolian sand (Table 1). Horizons in these soils (at the Mansion Beach site) were not included in the coarse textured grouping because the landscape setting differed distinctly from all other sites. The Mansion Beach site was located in an interdunal swale and the soils were uniform sand throughout. Horizons from all sites were placed in categories according to the abundance of concentrations and depletions described. Abundance categories follow the guidelines for describing RMFs found in the Soil Survey Manual (Soil Survey Staff, 1993): few equals <2%, common equals 2 to 20%, many equals >20%. Depletions and depleted matrices were defined as features or matrices with a chroma of 2 or less, and value of 4 or more. The depleted matrix definition follows the gleyed horizon criteria in the Soil Survey Manual (Soil Survey Staff, 1993). Combinations of concentrations and depletions in varying degrees of abundance within a horizon, including depleted matrices, were combined into six categories: (i) no RMFs, (ii) <2% RMFs, (iii) <2% depletions with >2% concentrations, (iv) 2–20% depletions with concentrations, (v) >20% depletions with concentrations, and (vi) depleted matrix with or without concentrations.

Water table hydrographs were developed using biweekly water table data, the maximum water table reading recorded between site visits, seepage rates (the rate the water table recedes), and daily precipitation data. Cumulative saturation values calculated using these hydrographs were within 3% of actual values (Morgan and Stolt, 2004). Precipitation data were collected at a weather station <5 km from each site. Average monthly precipitation recorded at the weather station between 1961 and 1990 were used to estimate 30 and 70% precipitation probabilities of more and less rainfall. The probabilities were calculated using a 2-parameter distribution (SCS, 1985; NRCS, 1995; SAS Institute, 1999). For this 2-yr period, 6 of the 24 mo had precipitation amounts below the 30% probabilities and 5 mo were above the 70% probability precipitation levels (Table 2). Long-term average annual precipitation on Block Island for the period of 1961–1990 was 103 cm. Annual precipitation during the monitoring period was 111 cm for September 2000 through August 2001, 87 cm for the period of September 2001 through August 2002, and within the sum of the precipitation defined by the 30 and 70% probability boundaries.

View larger version (30K): [in this window] [in a new window]   Fig. 1. Water table hydrographs for the Clayhead 2, Turnip Farm 19, and Mansion Beach 5 wells. The average seasonal high water table (ASHWT) depth for Clayhead 2, Turnip Farm 19, and Mansion Beach 5 wells are 44, 66, and 48 cm, respectively.

	RESULTS

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Soil Characteristics
The soils investigated during this study formed in a variety of glacial and Holocene parent materials (Table 1). Soils within the glacial deposits were classified to the subgroup level as Oxyaquic Quartzipsamments and Aquic, Oxyaquic, and Typic Dystrudepts. Textures ranged from silt to gravelly loamy coarse sand (Table 3). Mean bulk density values ranged from 1.17 for A horizons to 1.9 g cm^–3 for Cd horizons. Clay contents ranged from 1 to 18%; but averaged 6% (data not reported). Redoximorphic features were observed in all the soils. Coarse textured horizons generally had brighter matrix colors and fewer redox depletions than adjacent loamy textured horizons. Depleted matrices were present only in loamy textured C horizons. Soils formed in the sand dune deposits at the Mansion Beach site classified as Oxyaquic Quartzipsamments and Typic Psammaquents. Concentrations were fairly consistent in regards to size, color, and abundance throughout the Mansion Beach soils, and depletions were absent.

The range of annual fluctuation (highest recorded water table level to lowest recorded water table level in 1 yr) varied between wells (Table 1). Wells in which the water table was present for the entire study (Mansion Beach 4 and 5, Playground 10) had annual fluctuations between March and September 2001 of 120, 92, and 91 cm, respectively. Annual fluctuations in the same wells during 2002 were 60, 43, and 78 cm. The difference in annual fluctuation between years is attributed to 8 cm above 30-yr normal precipitation totals during the 2000–2001 monitoring period and 16 cm below the 30-yr normal precipitation totals for the 2001–2002 monitoring period. Where water tables dropped below the bottom of the wells, maximum lows were not recorded. However, water tables were recorded in some of the soil pits excavated to depths below the bottom of the wells (Clayhead 2 and 3, Turnip Farm 19 and 20, Playground 9). Soil pits were dug during July through September when water tables would be at or near their lowest levels. Water tables recorded in these pits were used as lowest water table levels recorded (Table 1).

Within the broad seasonal fluctuations described above, water table levels in all wells fluctuated in response to individual precipitation events. In general, the water table rose rapidly following a precipitation event, reaching its maximum level within the span of a few hours to a day. In the absence of subsequent precipitation events the water table then receded to its previous level over the course of the next few days to a week. These "spikes" in the water table vary in magnitude among sites and precipitation events and the cumulative effect over a season may be considerable. A representative range in patterns of these spikes for the soils we studied is illustrated in Fig. 1. For example, the water table at Clayhead 2 commonly rises 60 cm or more in response to a rainfall event, while at Mansion Beach 5 the water table rise is generally < 20 cm. Turnip Farm 19 shows an intermediate response to a precipitation event.

	DISCUSSION

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Soil Morphology–Water Table Relationships
One common approach to investigate soil morphology–water table relationships is to establish how well RMFs correspond to the depth of the SHWT. One problem with this approach is that there is not a standard method to define a SHWT. The difficulty can be seen in a typical hydrograph where the depth of the water table varies widely during the wet season and that these variations change from year to year (Fig. 1). To account for this fluctuation, some researchers have used monthly averages to define seasonal water table levels. Genthner et al. (1998) defined the SHWT as the average depth to the water table during the month of March, averaged over the 3 yr of the study. In a study on the coastal plain of Maryland, Galusky et al. (1998) aggregated 5 yr of water table measurements, taken four to five times monthly, to produce averages monthly water table levels. In this study, we defined the ASHWT as the average of the highest and lowest water table depths recorded during the two and a half month period of each year when water tables were at their highest levels.

Simple linear regression was used to compare the ASHWT depth and the depth to the first horizon of each RMF abundance category for 16 of the 20 sites (Fig. 2 ). The ASHWT for Clayhead 1, Claire 16, and Goldstein 18 were not calculated because the water table was often below the bottom of the well during the time of year when the ASHWT was calculated, or in the case of the Playground 8 well, below the bottom of the well for the duration of the study. When sites that the ASHWT could be calculated were considered together, the strongest correlation (r² = 0.42) was found between the depth to the ASHWT and the depth to the first horizon with common RMFs (Fig. 2a). This relationship improved (r² ranged from 0.65 to 0.72) when horizons in the coarse texture group, and those from the Mansion Beach site, were removed from consideration leaving only the loamy horizons (Fig. 2b, 2c, and 2d). This is illustrated by the higher r² values along with the closeness of the best-fit line to the line representing a 1:1 relationship. The regression line is essentially the same regardless of whether concentrations or depletions, of common abundance, are considered for loamy horizons. The strongest relationship (r² = 0.72) is obtained when the depth to the first horizon with common depletions is compared to the depth of the ASHWT (Fig. 2d). However, the relationship is also strong (r² = 0.65) when the depth to the first horizon with common concentrations or common depletions is considered (Fig. 2b). How the correlations between ASHWT and depth to certain RMF would change if data from all 20 sites were available is uncertain.

View larger version (25K): [in this window] [in a new window]   Fig. 2. Relationships between the depth to the average seasonal high water table (ASHWT) and depth to redoximorphic features (RMFs). Graph A shows the relationship between the depth to the ASHWT and the first horizon with >2% RMFs with 16 of the 20 wells considered together (regardless of texture). The ASHWT was not calculated for four of the sites because the water table was either often below the bottom of the well during the time of year the ASHWT was calculated or was never present in the well over the duration of the study. Graphs B, C, and D show the relationship between the ASHWT and RMFs for loamy textured horizons. Graph B shows the relationship between the depth to the ASHWT and the depth to the first loamy horizon with either >2% concentrations or >2% depletions. Graph C shows the relationship between the depth to the ASHWT and the depth to the first loamy horizon with >2% concentrations but <2% depletions. Graph D shows the relationship between the depth to the ASHWT and the depth to the first loamy horizon with >2% depletions. The dashed line represents a 1:1 relationship.

View larger version (27K): [in this window] [in a new window]   Fig. 3. Graph of the cumulative saturation with depth for the Turnip Farm 20 site. Shaded areas represent horizons with described redox features. At this site the soil was saturated above the shallowest expression of redox features for roughly 13% (67 d) of the 18 mo-long study period.

View larger version (11K): [in this window] [in a new window]   Fig. 4. Boxplots showing the relationship between cumulative saturation and the abundance of redoximorphic features in loamy textured horizons for the 18 mo-long study period. The three outliers from the 66 data points were removed before statistical analysis. A one way analysis of variance and Tukey's multiple comparison tests were used to determine differences between means. Means of boxplots with different letters are significantly different at the 0.05 level. Values below the x axis indicate the number of horizons having this class of redoximorphic feature (RMF). * indicates outliers.

View larger version (12K): [in this window] [in a new window]   Fig. 5. Boxplots showing relationship between cumulative saturation and the abundance of redox features in coarse textured horizons for the 18 mo-long study period. There was no significant difference between categories at the 0.05 level. Values below the x axis indicate the number of horizons having this class of redoximorphic feature (RMF).

In general, as the percentage of time horizons were saturated increased so did the abundance of RMFs (Fig. 4 and 5). Within the loamy texture group, cumulative saturation increased significantly in horizons with depletions in common or greater abundance when compared with horizons with few RMFs (Fig. 4). Cumulative saturation for loamy textured horizons with common depletions ranged from 5 to 56% (mean 22%) of the study. It should be noted that the percentage of cumulative saturation would likely have been less if we monitored for a full 2 yr, as the late summer and fall of the second year were not monitored. Only two coarse-textured horizons had common depletions. Cumulative saturation within these horizons was 32 and 57% (mean 45%).

Only concentrations were observed in the Mansion Beach profiles, and their abundance did not appear to be related to the cumulative saturation (Fig. 6 ). The cumulative saturation for horizons with few RMFs in these soils ranged from 4 to 64% (mean 30%), while the average cumulative saturation in horizons with common concentrations was 23% and ranged from 6 to 84%. The fact that the Mansion Beach soils are developing in eolian sands, are uniform in color and texture with depth, and have multiple buried surface horizons suggests that these soils are relatively young compared with the other soils investigated. A soil examined at the Mansion Beach site where water table data indicated well-drained conditions (the well was installed at a much later date than the study wells and therefore the data not reported), revealed morphology similar to that of the more poorly drained soils on the site. The lack of B horizon development (specifically in regard to color) at this well drained site suggests that the uniform low chroma colors (2.5Y 5/3, 6/3) throughout the matrix are not due to wetness. At Mansion Beach, the depth to the SHWT is best estimated by the depth to the shallowest occurrence of concentrations.

View larger version (10K): [in this window] [in a new window]   Fig. 6. Boxplots showing relationship between cumulative saturation and the abundance of redox features at the Mansion Beach site for the 18 mo-long study period. There was no significant difference between categories at the 0.05 level. Values below the x axis indicate the number of horizons having this class of redoximorphic feature (RMF). * indicates outliers.

Depleted matrices or many depletions were only found in loamy textured horizons. The cumulative saturations for horizons with many depletions averaged 26%, and ranged from 11 to 38%. Depleted matrices had the longest mean cumulative saturation (37%) out of all the categories. A depleted matrix, however, was not always the best indicator of a relatively high cumulative saturation. In the Playground 9 (glaciofluvial) soil, the water table regularly fluctuated in and out of a horizon with a chroma two matrix during March and April of 2001, but over the duration of this study the water table was recorded within this horizon for only 5% of the time. This 2Cg horizon was silt loam in texture and located directly above a coarse sand horizon (Table 3). Because the finer-textured horizon must become saturated before water will move from it into the coarser-textured material below, the finer-textured material can remain at or near saturation even though the horizon below is not saturated (Clothier et al., 1978; Vepraskas and Guertal, 1992). This juxtaposition of textures and the associated RMFs commonly leads to misinterpretation of the SHWT (Vepraskas et al., 1974; Veneman et al., 1976; Clothier et al., 1978). When this soil was described and sampled, the silt loam horizon gave up free water when squeezed. The high moisture content was quite surprising considering the top of the water table was approximately 50 cm below the bottom of the horizon, the last significant precipitation event (1.4 cm) occurred 20 d prior, and the well data showed the water table had not reached that horizon for over 60 d. We installed a shallow well at this location to determine if episaturation was occurring at the site. A water table was observed in the shallow well following 4.4 cm of rain on one occasion, while the water table in the adjacent deep well was much deeper. These observations indicate that on occasion episaturation occurs within and above this silt loam horizon. Depending on conditions, this site can have endosaturation, episaturation, or near saturation. Therefore, this horizon is probably reduced for a longer duration than the 1 to 5% saturation represented by the measurement from the deeper well. The increased period of reduction likely equates to the low chroma matrix in a horizon where the water table was recorded for a relatively small amount of time.

Implications
The strong relationship (r² ranged from 0.65 to 0.72) between the ASHWT and the depth to the shallowest loamy horizon with common RMFs suggests the identification of these features provides a good place to start when making SHWT interpretations in loamy soils. Where coarse textured horizons are present, additional consideration should be given to soil-based interpretation of the depth to the SHWT. Our studies support others that suggest that the abundance and type of RMFs are related to the amount of time the water table is present within a horizon. In loamy textured horizons, cumulative saturation was significantly different between those horizons with few RMFs and those with common or greater depletions. Coarser-textured horizons were less expressive in regards to abundance of depletions than were loamy textured horizons with similar cumulative saturation. For example, the average cumulative saturation for coarse-textured horizons with few and common RMFs (32%) was similar the mean cumulative saturation of loamy horizons with many depletions or a depleted matrix (26 and 36%, respectively). This suggests that soil texture should be considered when making interpretations from RMFs.

Regulatory agencies often use abundance of RMFs as part of the criteria for making SHWT determinations for land use decisions such as on-site waste disposal (OSWD). In Rhode Island, one of the criteria for determining the depth to the SHWT for OSWD siting is the presence of common RMFs (RIDEM, 2000). We found the SHWT was often above the shallowest horizon with common RMFs, and either within horizons with few RMFs, or in horizons with no RMFs. During this study, the water table was recorded in horizons (excluding A horizons) above the shallowest occurrence of common RMFs in 13 of 17 wells in loamy soils. The cumulative saturation above common RMFs in these wells ranged from <1 to 13% (mean 6%). In a coarse textured soil, such as Turnip Farm 19, the first occurrence of common RMFs had an average cumulative saturation of 21% (Table 3). If the OSWD treatment zone is meant to be the soil layers between the bottom of the drainfield and the depth to common RMFs, the treatment zone for this wastewater disposal system would be compromised 21% of the time. The depth to 2 or less chroma depletions is commonly taught as an indicator of SHWT throughout the eastern United States (Karathanasis et al., 1996; Pennsylvania State University, 2002). Because depletions indicated much greater cumulative periods of saturation than concentrations, the use of depletions to define the lower limit of the treatment zone may result in an interpretation of the SHWT where even a higher amount of time the treatment zone is compromised.

Soil morphology is a valuable tool for interpreting water table activity. The use of this tool should be broadened to not only provide an estimate of the depth of a defined SHWT but also the cumulative duration of the water table within a horizon.

	ACKNOWLEDGMENTS

 
This work is a contribution (No. 4092) of the Rhode Island Agricultural Experiment Station. This work was funded through the U.S. EPA Block Island and Green Hill Pond Watershed National Decentralized Wastewater Treatment Demonstration Project. The authors thank Dr. Tim Tyrrell for assistance in calculating the 2-parameter gamma distribution data.

Received for publication February 23, 2004.

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