Wetland Hydrology Determinations for the 2017 Monitoring Season
The Norfolk District believes it beneficial to the public to provide seasonal Public Notices (PNs) regarding preceding precipitation conditions for shallow groundwater well monitoring associated with wetland determinations. The purpose of this PN is to inform the public of relevant parts of our process, and our interpretation and findings regarding current precipitation conditions for the 2017 monitoring season. This is accomplished by our analysis of several precipitation recording stations and our use of groundwater monitoring wells at different reference locations in the Hampton Roads region.
For the three-month antecedent ending in February, 2017, precipitation totals for southeastern Virginia were below normal ranges in Norfolk [Norfolk International Airport], Suffolk [Lake Kilby], and Hampton [Joint Base Langley-Eustis], and within the normal range for Williamsburg [Williamsburg 2N]. For the three-month antecedent ending in March, 2017, precipitation totals for the Norfolk, Suffolk, and Williamsburg stations were all within the normal range, with the Hampton station below the normal range. We have reviewed both the current and antecedent precipitation conditions, the distribution of the precipitation, and groundwater data at reference wetland sites. We conclude that groundwater monitoring well data, indicating wetland hydrology does not meet the technical standard, is likely unreliable for wetland determinations this year.
Wetlands are defined as "those areas that are inundated or saturated by surface or groundwater at a frequency and duration sufficient to support, and that under normal circumstances do support, a prevalence of vegetation typically adapted for life in saturated soil conditions" (33 CFR 328.3(b)(emphasis added). Wetland determinations typically entail observation of field indicators of wetland vegetation, hydric (wetland) soils, and wetland hydrology.
Occasionally, property owners or their agents may elect to install and monitor shallow groundwater wells for the late winter and spring seasons to gather data about the levels and duration of groundwater (i.e. saturated soil conditions) for particular areas to attempt to clarify the limits of wetlands. There is no requirement that well data be submitted to obtain confirmations of wetland delineation. However, the Corps does consider groundwater well data in its determinations if such data are collected in accordance with proper well installation and monitoring standards, and during periods consistent with "normal circumstances" prior to and during the monitoring period.
Any monitoring wells used to facilitate wetland hydrology determinations should be installed and monitored in accordance with the guidelines in Technical Standard for Water-Table Monitoring of Potential Wetland Sites, ERDC-TN-WRAP-05-2.
Before we will consider well data for a specified site, we require submittal and approval of a well monitoring plan, which includes a review of the location and installation of the monitoring wells. In addition, during the monitoring season (typically February through April), the Corps' staff must be allowed periodic access to the particular sites and wells without any prior notice to provide proper quality assurance.
When reviewing shallow groundwater well data in order to determine whether normal circumstances for wetland hydrology are present, we consider the amount and distribution of precipitation prior to the start of the growing season (after leaf drop in the fall) and during the early growing season. The U.S. Department of Agriculture’s Natural Resource Conservation Service (NRCS) National Water and Climate Center calculates normal precipitation ranges for each month (defined as between the 30th and 70th percentiles of monthly precipitation totals) for NOAA’s National Weather Service (NWS) stations throughout the United States. The information for Virginia is published in WETS tables available from the NRCS.
Methods to evaluate precipitation trends for wetland delineation purposes are described in Accessing and Using Meteorological Data to Evaluate Wetland Hydrology, ERDC TR-WRAP-00-1. Short-term water-table monitoring data (i.e., <10 years) must be evaluated with consideration of the amount and distribution of precipitation that fell prior to the beginning of the growing season (but after leaf drop in the fall). Although we analyze all months after leaf drop, this timeframe is generally at least 3 months prior to the beginning of the growing season each year.
Monthly precipitation totals for the three (3) months (Jan-Feb-March) preceding this PN for southeastern Virginia have been within normal ranges (summary data attached for Lake Kilby, Langley, Williamsburg & Norfolk stations). As mentioned above, the precipitation totals for the previous 3 months (Dec-Jan-Feb) was below the normal range for all but the Williamsburg station. Precipitation trends this year included very low quantities from late January through mid-March, with an uneven distribution of precipitation in March. While March rainfall totals were within the normal range (slightly above normal for Suffolk), approximately 80% of the monthly rainfall fell over three days (13th, 14th, and 31st) due to winter storm Stella and another storm event. As with PNs on this topic issued for previous years, this assessment only analyzes whether precipitation total amounts for the prior three months fall within the 30th to 70th percentiles for the start of the growing season.
As the growing season progresses precipitation conditions will be similarly analyzed until full leaf-out, generally around mid- to late April. Dry wells in a drier-than-typical precipitation period and wet wells in a wetter-than-typical precipitation period are of limited value in making definitive determinations regarding wetland hydrology.
This analysis is based on evaluation of precipitation data for four WETS stations in southeastern Virginia (Norfolk, Lake Kilby, Langley, & Williamsburg). Precipitation trends (e.g., percent of normal for different time periods) for the entire Commonwealth are available from NWS’s Advanced Hydrologic Prediction Service, and they correspond well with our more detailed analyses for Tidewater, Virginia.
Lastly, we have reviewed water levels in several shallow groundwater reference well sites in southeastern Virginia which have been monitored for several years. Water levels at all sites were lower than typically observed during seasons with normal precipitation conditions, particularly during the timeframe of late January through mid-March. Several of the monitoring wells which met the wetland hydrology standard in many years past have not yet met the standard this year (water table is less than or equal to 12 inches below the soil surface for greater than or equal to 14 consecutive days during the growing season).
While groundwater levels have risen to within 12 inches of the soil surface for many of the reference wells, this trend was not sustained long enough to meet the hydrology technical standard. In summary, lower than normal groundwater levels and shorter duration of seasonal high water levels were observed in the reference wells this season. While precipitation quantity and distribution is the main factor, warm temperatures in February and an early growing season start may also be contributing factors. Representative hydrographs from several reference sites are enclosed.
It’s important to note that the precipitation evaluations described above are based on monthly totals, so the distribution of precipitation events within a particular month are not factored in even though it can affect rates of infiltration and observed groundwater levels. Additionally, there are limitations in making predictions about wetland hydrology by simply evaluating trends of monthly precipitation totals. For example, this evaluation does not factor in air temperatures which can greatly influence evapotranspiration rates of evergreen plants, which in turn can affect seasonal water table levels.
Also, direct precipitation is not the primary water source for all wetlands; different wetland classes have different primary sources of sustaining hydrology. These differences highlight the importance of having multiple years of hydrology observations, and why the Corps’ technical standard for wetland hydrology includes a minimum frequency interval of 5 years in 10 (≥50% probability). Comparison of hydrographs over several years from the same site can help identify when precipitation alone may not be an accurate surrogate for assessing overall wetland hydrology.
Regardless of precipitation conditions, we will continue to make wetland determinations based on the field indicators of vegetation, soils, and wetland hydrology described in the Regional Supplement to the Corps of Engineers Wetland Delineation Manual: Atlantic and Gulf Coastal Plain Region (version 2.0), the Regional Supplement to the Corps of Engineers Wetland Delineation Manual: Eastern Mountains and Piedmont Region (version 2.0), including use of Chapter 5 when appropriate, and portions of the Corps of Engineers Wetlands Delineation Manual (1987) that were not replaced by the regional supplements. Chapter 5 (Difficult Wetland Situations) of both Regional Supplements applicable in Virginia contain a section addressing “Wetlands that periodically lack indicators of wetland hydrology”. This section describes a number of approaches that can be used to determine whether wetland hydrology is present on sites where indicators of hydrophytic vegetation and hydric soils are present but hydrology indicators may be lacking due to factors including periods with below-normal rainfall conditions. Considering the current and antecedent precipitation trends, use of this portion Chapter 5 may be appropriate.
Beginning and ending dates of the growing season are needed in the event water-table monitoring data must be analyzed for wetland hydrology determinations. The Regional Supplements state that the growing season has begun and is ongoing in a given year when two or more different non-evergreen vascular plant species growing on the site or surrounding areas exhibit certain indicators of biological activity, or when soil temperature measured at the -12-in. (-30-cm) depth is 41 °F (5 °C) or higher. Soil temperature and plant activity are both surrogates for estimating soil microbial activity which produces anaerobic conditions in the soil that are characteristic of wetlands.
Based on soil temperature data collected at several reference sites in southeastern Virginia over the past 7 years, soil temperatures typically remain above 41 °F for most of the year. A graph of soil temperature data collected at reference sites in southeastern Virginia (Virginia Beach, Chesapeake, York County and Isle of Wight County) from approximately November 2016 to March 2017 is attached. The data collected indicates soil temperatures remained well above 41 °F for the entire monitoring period for all reference sites.
Several of the indicators of plant biological activity signaling the start of the growing season as described in the Regional Supplements were observed broadly across southeastern Virginia as early as the first full week of February 2017. We consider this to be the start date of the growing season in southeastern Virginia. However, we will give consideration to any site-specific data submitted that supports a different start date.
This PN does not relieve those that have constructed wetland mitigation projects from monitoring hydrologic conditions. Monitoring should be conducted in accordance with the associated permit, approved plan, or mitigation banking instrument.