Hydrobiogeochemical Controls on Riparian Nutrient and Greenhouse Gas Dynamics: 10 Years Post‐Restoration

Little is known about the impact of agricultural legacy on subsurface biogeochemical processes in the years following restoration of riparian wetlands (WLs). More knowledge is also needed on the relative importance of seasons, precipitation events, and inputs of water and nutrients driving nitrogen (N), phosphorus (P), sulfur (S), and greenhouse gas (GHG) (N₂O, CO₂, CH₄) dynamics in these systems. This investigation of a riparian zone comprising a restored WL area and a nonrestored well‐drained alluvium (AL) area in the United States Midwest revealed that despite successful hydrological restoration a decade earlier, biogeochemical conditions in the WL area remained less anoxic than in natural WLs, and not significantly different from those in the AL area. No significant differences in N, P, S, and C compound concentrations or fluxes were observed between the AL and WL areas. Over the duration of the study, nitrate (NO₃^?) and soluble reactive phosphorus appeared to be primarily driven by hillslope contributions. Ammonium (NH₄⁺), sulfate (SO₄^2?), and CO₂ responded strongly to seasonal changes in biogeochemical conditions in the riparian zone, while N₂O and CH₄ fluxes were most influenced by large rewetting events. Overall, our results challenge overly simplistic assumptions derived from direct interpretation of redox thermodynamics, and show complex patterns of solutes and GHGs at the riparian zone scale.     

Groundwater Nitrate Concentration Reductions in a Riparian Buffer Enrolled in the NC Conservation Reserve Enhancement Program

Riparian buffers have been used for many years as a best management practice to decrease the effects of nonpoint pollution from watersheds. The NC Conservation Reserve Enhancement Program (NC CREP) has established buffers to treat groundwater nitrate‐nitrogen (NO₃^?‐N) from agricultural sources in multiple river basins. A maturing 46 m wide riparian buffer enrolled in NC CREP was studied to determine its effectiveness in reducing groundwater NO₃^?‐N concentrations from a cattle pasture fertilized with poultry litter. Three monitoring blocks that included groundwater quality wells, water table wells, and soil redox probes, were established in the buffer. NO₃^?‐N concentrations decreased significantly across the buffer in all of the monitoring blocks with mean reductions of 76‐92%. Many biological processes, including denitrification and plant uptake, may have been responsible for the observed NO₃^?‐N reductions but could not be differentiated in this study. However, mean reductions in Cl^? concentrations ranged from 48‐65% through the blocks, which indicated that dilution was an important factor in observed NO₃^?‐N reductions. These findings should be carefully considered for future buffer enrollments when assigning nitrogen removal credits.     

TIMBER HARVEST IMPACTS ON SMALL HEADWATER STREAM CHANNELS IN THE COAST RANGES OF WASHINGTON1

ABSTRACT: We evaluated changes in channel habitat distributions, particle‐size distributions of bed material, and stream temperatures in a total of 15 first‐or second‐order streams within and nearby four planned commercial timber harvest units prior to and following timber harvest. Four of the 15 stream basins were not harvested, and these streams served as references. Three streams were cut with unthinned riparian buffers; one was cut with a partial buffer; one was cut with a buffer of non‐merchantable trees; and the remaining six basins were clearcut to the channel edge. In the clearcut streams, logging debris covered or buried 98 percent of the channel length to an average depth of 0.94 meters. The slash trapped fine sediment in the channel by inhibiting fluvial transport, and the average percentage of fines increased from 12 percent to 44 percent. The trees along buffered streams served as a fence to keep out logging debris during the first summer following timber harvest. Particle size distributions and habitat distributions in the buffered and reference streams were largely unchanged from the pre‐harvest to post‐harvest surveys. The debris that buried the clearcut streams effectively shaded most of these streams and protected them from temperature increases. These surveys have documented immediate channel changes due to timber harvest, but channel conditions will evolve over time as the slash decays and becomes redistributed and as new vegetation develops on the channel margins.     

CHANGING NEAR‐STREAM LAND USE AND RWER CHANNEL MORPHOLOGY IN THE VENEZUELAN ANDES1

ABSTRACT: The shape of a river channel is linked to surrounding land use through interacting hydrologic and geologic processes. This study analyzes the relationship between the change in near‐stream land use and the shape of the adjacent river channel over time. Three watersheds in the foothills of the Venezuelan Andes that have experienced differing degrees of development were studied to determine river channel width, sinuosity, and position relative to surrounding land use. Change in land use over time was obtained from multiple‐date aerial photographs (1946 and 1980) referenced to 1996 Landsat Thematic Mapper (TM) satellite imagery, and verified by field inspection. Measurements of land‐use type and amount and river channel morphology from the two dates were made using geographic information system (GIS) methods. The three watersheds differed in the extent of deforestation, the location of remaining forested land, and how much land‐use change had already occurred by 1946. Change in river channel morphology was greatest at the most deforested sites. Valley shape and channel constraint also had a discernible effect on change in channel morphology. This study introduces a method for analyzing change in coupled terrestrial‐aquatic systems based on multiple‐date, remotely sensed data and GIS analysis of spatial properties. The results document human impacts on river channels through a comparison of multiple watersheds over a 35‐year time interval.     

ANALYZIIG RIPARIAN SITE CAPABILITY AND MANAGEMENT OPTIONS1

ABSTRACT: Riparian areas interact with aquatic and upland conditions and therefore help determine the degree of functionality (streambank stability, shade, sediment, and debris filtering) found in a watershed or catchment. Thus, conditions in riparian areas exert significant influence on water quality. Physical and biological factors (biophysical determinants) that influence these conditions and determine long‐term site ecology include topographic variables, geology, climate, soil texture, and others. These conditions are further modified by management infrastructure (roads, dikes, etc.). Our objective was to develop a system for evaluating site condition in relation to site capability. Since biophysical determinants and infrastructure interact with water quality, our first task was to acquire data concerning the spatial distribution of biophysical determinants and infrastructure constraints and to import them into a GIS system where they could be managed and processed. To expedite analysis, determinants and infrastructure constraints were placed into a hierarchy capable of isolating various site capability types. The hierarchy was designed to incorporate multiscale effects. Site capability areas are georeferenced in this process thereby enabling efficient monitoring and providing a way to focus management on those areas needing improvement. Study tasks included: (1) landscape characterization and hierarchy selection, (2) field assessment, (3) information management and data mining, and (4) information interpretation and adaptive management. This approach appears to be an effective way to isolate general ripananstandardsmaycon site conditions, to provide indications about water quality, and to create strategies necessary for alleviating water quality problems.     

TRANSPORT AND FATE OF ATRAZINE IN MIDWESTERN RIPARIAN BUFFER STRIPS1

ABSTRACT: The fate of pesticides entering the Riparian Buffer Strips (RBS) has not been well documented. This study compared the transport and fate of atrazine in soil of three‐, five‐, and nine‐year‐old switchgrass (Panicum virgatum L.) RBS to that in adjacent soils cropped to a corn‐soybean rotation or a grass‐alfalfa pasture. Undisturbed soil columns were collected from the RBS and cropped areas within the Bear Creek watershed, near Roland, Iowa. Atrazine and bromide breakthrough curves obtained using intact soil columns under saturated conditions were described by a two‐region, mobile‐immobile transport model. Preferential flow of bromide and atrazine was evident in five‐and nine‐year‐old RBS soil, but there was little difference in transport characteristics between these two RBS soils and the adjacent cropped soils. There was a trend towards an increase in dispersion coefficients between the five‐and nine‐year‐old RBS sites, which suggests an increased degree of preferential flow with increasing RBS age. Despite similar texture and organic C contents, atrazine sorption was significantly greater in RBS soil than the adjacent cropped soil. Cropped soil degraded atrazine faster than the RBS soil. The rapid degradation of atrazine in the corn‐soybean soil adjacent to the five‐year‐old RBS (atrazine half‐life of 19 days) appeared to be due to a larger population of atrazine‐degrading microorganisms. Atrazine‐degrading microorganisms in the corn‐soybean soil were 50,940 cells g^‐1 soil compared with 2,970 cells g^‐4 soil in 5‐year‐old RBS soil which resulted in 60 percent mineralization of [¹⁴C‐UL‐atrazine] in the corn‐soybean soil.     

EVALUATION OF COASTAL PLAIN CONSERVATION BUFFERS USING THE RIPARIAN ECOSYSTEM MANAGEMENT MODEL1

ABSTRACT: Riparian buffers are increasingly important as watershed management tools and are cost‐shared by programs such as Conservation Reserve that are part of the USDA Conservation Buffer Initiative. Riparian buffers as narrow as 4.6m (15ft) are eligible for cost‐share by USDA. The Riparian Ecosystem Management Model (REMM) provides a tool to judge water quality improvement by buffers and to set design criteria for nutrient and sediment load reduction. REMM was used for a Coastal Plain site to simulate 14 different buffers ranging from 4.6 m to 51.8 m (15 to 170 ft) with three different types of vegetation (hardwood trees, pine trees, and perennial grass) with two water and nutrient loads. The load cases were low sediment/low nutrient‐typical of a well managed agricultural field and low sediment/high nutrient‐typical of liquid manure application to perennial forage crops. Simulations showed that the minimum width buffer (4.6 m) was inadequate for control of nutrients under either load case. The minimum width buffer that is eligible for cost share assistance on a field with known water quality problems (10.7 m, 35 ft) was projected to achieve at least 50 percent reduction of N, P, and sediment in the load cases simulated.     

浑太河流域多尺度土地利用/景观格局与水质响应关系初步分析

选取浑太河流域为研究对象,采用GIS、相关性分析、多元线性回归分析等多种数理统计分析技术,从子流域和河岸缓冲区尺度分析了土地利用、景观格局对河流水质的影响。结果表明:农业用地面积比例在子流域和1000m河岸缓冲区尺度与NH4+-N、TN、BOD5和COND呈正相关;林地用地面积比例在子流域尺度和河岸缓冲区尺度均与CODMn呈负相关,在300m、400m、1000m河岸缓冲区尺度与TN呈负相关,在1000m河岸缓冲区尺度与COND呈负相关,在子流域与1000m河岸缓冲区尺度与NO3-呈正相关;草地在子流域尺度与NH4+-N、TN、TP、Chl、CODMn呈正相关;建设用地面积比例在子流域尺度与BOD5、COND、TN呈正相关。CONTAG在河岸带缓冲区尺度与COND、TN呈正相关,在子流域和河岸缓冲区尺度与均CODMn均呈正相关,在河岸缓冲区尺度与NO3-呈负相关;PD在子流域尺度上与TN呈正相关,ED在子流域尺度上与NO3-呈正相关,与NH4+呈负相关;SHDI在子流域与河岸缓冲区尺度与NO3-均呈负相关。相对于河岸缓冲区尺度,土地利用类型在子流域尺度上对水质的影响更为显著,表现在调整判定系数Adjusted R2更大。而景观格局指数中对水质影响最大的是CONTAG,相对于子流域尺度,CONTAG在缓冲区尺度上影响更为显著。     

碳化条件下混凝土中钢筋腐蚀pH阈值研究

目的 确定碳化条件下几种典型缓冲溶液及模拟混凝土孔隙液中钢筋的腐蚀pH阈值范围。方法 采用电化学阻抗谱法,对不同pH、不同配制方法的缓冲溶液及模拟混凝土孔隙液中的钢筋试片进行电化学测量,选用不同的等效电路进行拟合,对测量中钢筋试片发生的电化学过程进行简要阐述。结合阻抗谱测量结果、等效电路拟合结果及工作电极典型宏观形貌,判断钢筋试片腐蚀状态,给出不同溶液对应的钢筋腐蚀pH阈值范围。结果离子环境不同,各缓冲溶液及模拟混凝土孔隙液中钢筋腐蚀pH阈值均存在一定差异,位于9～10.5不等。结论 混凝土碳化引入的HCO₃~–和CO₃^2–对钢筋腐蚀有促进作用。混凝土孔隙液pH越低,钢筋腐蚀越容易发生。因此,仅根据pH判断混凝土中钢筋腐蚀状态的方法并不可靠。     

河岸芦苇、茭白和香蒲植物带处理受污染河水中试研究

利用中试规模的河岸植物带(芦苇、茭白、香蒲)及无植物空白带对受污染河水进行处理,并对它们处理结果进行比较.结果表明,3种植物均能强化污染物的去除.其中芦苇带效果最好,它在整个运行期间对COD、NH₄⁺-N和TP的去除效果分别为43.7%、79.5%和75.2%.对出水溶解氧与水温进行了考察,结果表明植物带水体的溶解氧变化比无植物带小,水温比无植物带低.其中,芦苇带水的溶解氧最高,香蒲带水温和溶解氧最低.芦苇、茭白、香蒲带对河水中污染物的去除和改善水环境条件等方面表现了不同的特点,在河岸带修复时应尽可能保证水生植物的多样性.