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1.
Large water‐sample sets collected from 1899 through 1902, 1907, and in the early 1950s allow comparisons of pre‐impoundment and post‐impoundment (1969 through 2008) nitrogen concentrations in the lower Missouri River. Although urban wastes were not large enough to detectably increase annual loads of total nitrogen at the beginning of the 20th century, carcass waste, stock‐yard manure, and untreated human wastes measurably increased ammonia and organic‐nitrogen concentrations during low flows. Average total‐nitrogen concentrations in both periods were about 2.5 mg/l, but much of the particulate‐organic nitrogen, which was the dominant form of nitrogen around 1900, has been replaced by nitrate. This change in speciation was caused by the nearly 80% decrease in suspended‐sediment concentrations that occurred after impoundment, modern agriculture, drainage of riparian wetlands, and sewage treatment. Nevertheless, bioavailable nitrogen has not been low enough to limit primary production in the Missouri River since the beginning of the 20th century. Nitrate concentrations have increased more rapidly from 2000 through 2008 (5 to 12% per year), thus increasing bioavailable nitrogen delivered to the Mississippi River and affecting Gulf Coast hypoxia. The increase in nitrate concentrations with distance downstream is much greater during the post‐impoundment period. If strategies to decrease total‐nitrogen loads focus on particulate N, substantial decreases will be difficult because particulate nitrogen is now only 23% of total nitrogen in the Missouri River. A strategy aimed at decreasing particulates also could further exacerbate land loss along the Gulf of Mexico, which has been sediment starved since Missouri River impoundment. In contrast, strategies or benchmarks aimed at decreasing nitrate loads could substantially decrease nitrogen loadings because nitrates now constitute over half of the Missouri's nitrogen input to the Mississippi. Ongoing restoration and creation of wetlands along the Missouri River could be part of such a nitrate‐reduction strategy. Published 2013. This article is a U.S. Government work and is in the public domain in the USA. 相似文献
2.
水库运行容易在库区诱发滑坡、岩溶塌陷等地质灾害。通过水库运行的安全调度，可有效减少库区地质灾害的发生。本文以在建的大藤峡水库武宣防护区为例，分析计算了岩溶塌陷预防的安全调度水位、排蓄水速度;并根据现场调查,查明了研究区岩溶塌陷发育的地质背景和发育现状，建立了岩溶塌陷发育机理模式。为了分析水库运行改变地下水动力条件对岩溶塌陷发育的影响，运用Modflow软件，建立研究区数值模拟模型，通过数值模拟运算，预测了大藤峡水库不同蓄水水位引起的地下水位变化，以及不同排蓄水速度引起的地下水流速变化；探讨了包气带、潜水层和承压水层区域岩溶塌陷发育的可能性。结果表明:研究区岩溶塌陷预防的水库安全水位高程为51～62 m,蓄水水位安全上升速度为0～1.0 m·d-1和排水水位安全下降速度为0~3.0 m·d-1。 相似文献
3.
利用2001～2015年三峡数字遥测地震台网记录到的数字地震观测资料，结合三峡工程水库蓄水情况，采用Atkinson方法分析该地区的地震波衰减特征，得到不同蓄水阶段、不同库段的非弹性衰减结果。结果显示，Q值与频率变化的线性关系较好，各频点在拟合线附近波动；3个库段在不同蓄水阶段均具有较小的Q0值和中等的η值，表明三峡工程水库重点监视区构造复杂，且介质的均匀程度较低；仙女山-九畹溪断裂附近地区Q值较大，表明该地区构造运动比较稳定；巴东高桥断裂附近地区Q值较小，表明该地区构造运动比较强烈；泄滩地区随着水位的升高，Q0值增大，表明该地区介质均匀程度有所提高。 相似文献
4.
??FG5??112?????????????й?????????????硱????????? ??JZ06????????4?ξ?????????????????????????????????????????????????????????????????????1??2001??2008??3?ι?????????????????20??10-8ms-2???????????????????????????????????????????15.4??10-8ms-2??????6.7??10-8??ms-2????2?????????仯??????????????????????????????????仯?????0.61??10-8ms????-2??/a???????????λ???????-0.18cm/a??????????????????仯?????????????????????????????????????????????????????? 相似文献
5.
长江三峡GPS处理结果和应变背景场 总被引：1，自引：0，他引：1
采用BerneseGPSSoftware 4 .2对长江三峡工程诱发地震监测系统GPS监测网络 1998～ 2 0 0 3年的 6期观测数据进行了处理 ,结果表明 ,三峡库区与华南块体的水平相对运动在 0～ 3mm/a(± 0 .1～± 2 .0mm/a) ;蓄水导致的垂直形变区域主要集中在茅坪 香溪 巴东库段 ,近岸点垂直沉降的量级在 10～ 35mm左右 ,最大峰值区域香溪约 35mm(± 8.6mm) ,垂直形变沿远离库中心方向迅速衰减。采用贝塞尔双三次样条函数模型拟合该地区的应变率 ,推算各类水平应变场 ,结果表明 ,蓄水前各种应变背景在 10 -9/a～ 10 -10 /a量级。作为构造稳定地区 ,三峡库区近期因蓄水导致大规模形变 ,从而诱发中强地震的可能性不大 相似文献
6.
主要探讨水稻田休耕期间蓄水对休耕后土壤肥力之影响。选择两块水稻田区,在水稻收割后,将田区翻耕一次。一田区以人工连续灌溉,蓄存水量于田区;另一田区则为天然蓄水。在每一田区四边及中央处,以10cm为一层采取土样,取至60cm。再将每一层土样气干后,通过0.42mm筛网土样混合,分析田区同一层在不同时段与不同深度下之pH值、电导度值、有机质、全氮、有效磷、钙离子、镁离子、钾离子及钠离子之变化。试验结果显示:休耕后水稻根系腐化,造成土壤有机质以及有效磷显著的增加。蓄水后天然蓄水田区有机质和有效磷分别增加51%及88%,连续蓄水田区分别增加13%和69%。而天然蓄水田区镁、钾、钠分别降低35%、24%和47%,连续蓄水田区分别降低29%、14%及41%。 相似文献
7.
R. W. Parkinson T. C. Wang J. R. White J. R. David M. E. Hoffman 《Environmental Geology》1993,22(1):26-32
This project was designed to: (1) document the distribution and migration of organochlorine pesticide residues within marsh substrates of 18 St. Lucie County mosquito control impoundments located along the Indian River Lagoon estuary, and (2) evaluate the impact of water management techniques on residue mobility. Our results indicate that detectible concentrations of organochlorine compounds, applied between the late 1940s and early 1950s, are present in 16 of the 18 St. Lucie County mosquito control impoundments. These compounds are primarily restricted to the surficial, organic-rich wetland sediment, which, based upon geotechnical analysis, was exposed to the atmosphere at a time when the impoundments were subjected to pesticide treatment. Contaminated sediments are present below the surficial, organic-rich layer, suggesting that some vertical migration of pesticides has occurred. It is unlikely that leaching associated with the downward percolation of impounded water was responsible for this migration as pesticide residues were never detected within thein situ pore waters. An alternative explanation is that biological processes (e.g., rooting, burrowing) facilitated the downward flux of organochlorine compounds into sediment horizons not subjected to direct treatment. Eighty-eight surface water samples obtained from two impoundments subjected to contrasting water management techniques were analyzed for pesticide content. None of the surficial water samples collected in association with these impoundments contained detectible concentrations of organochlorine compounds. These samples were unfiltered and contained as much as 25 mg/1 of particulate organic matter. This suggests that the currently preferred management technique (RIM), which is designed to maintain water quality, limit mosquito production, and provide for ecological continuity, does not hydraulically mobilize pesticide residues into the Indian River Lagoon estuary. 相似文献
8.
三峡水库蓄水前后长江中下游流量特征变化及其对造床作用的影响一直受到学者的关注.采用枝城等6个水文站日均流量资料,分别统计了各站流量的经验频率分布,检验了8种概率密度函数的适用性,并讨论了三峡水库蓄水前后流量频率分布特征与造床流量的关系.研究表明:长江中下游干流洪中枯各级流量的频率分布具有分段特性,无法用皮尔逊Ⅲ型或对数正态等常见函数来整体描述,但中洪流量区段符合分段幂函数;拟合得到的分段幂函数转折点可近似反映造床流量,包括临界值在内的函数特征参数受洞庭湖等调蓄作用影响,在螺山站上下游差异较大;三峡水库蓄水后,洪水削减使得各站的流量频率分段特性增强,分段幂函数仍然适用;由于三峡水库蓄水后流量过程变化和频率调整的影响,枝城、沙市、监利站造床流量将变为蓄水前的0.79、0.73、0.74倍,螺山、汉口、大通站造床流量将变为蓄水前的0.89、0.90、0.93倍.本文的方法和认识对于长江中下游径流特征分析和造床流量计算具有参考价值. 相似文献
9.
Seismic analysis of the Xiluodu reservoir area and insights into the geometry of seismogenic faults 
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下载免费PDF全文 The Xiluodu (XLD) reservoir is the second largest reservoir in China and the largest in the Jinsha River basin. The occurrence of two M > 5 earthquakes after reservoir impoundment has aroused great interest among seismologists and plant operators. We comprehensively analyzed the seismicity of the XLD reservoir area using precise earthquake relocation results and focal mechanism solutions and found that the seismicity of this area was weak before impoundment. Following impoundment, earthquake activity increased significantly. The occurrence of M ≥ 3.5 earthquakes within five years of impoundment also appear to be closely related to rapid rises and falls in water level, though this correlation weakened after five years because earthquake activity was far from the reservoir area. Earthquakes in the XLD reservoir area are clustered; near the dam (Area A), small faults are intermittently distributed along the river, while Area B is composed of multiple NW-trending left-lateral strike-slip faults and a thrust fault and Area C is composed of a NW-trending left-lateral strike-slip main fault and a nearly EW-trending right-lateral strike-slip minor fault. The geometries of the deep and the shallow parts of the NW-trending fault differ. Under the action of the NW-trending background stress field, a series of NW-trending left-lateral strike-slip faults and NE-trending thrust faults in critical stress states were dislocated due to the stress caused by reservoir impoundment. The two largest earthquakes in the XLD reservoir area were tectonic earthquakes that were directly triggered by impoundment. 相似文献
10.
Introduction During and before the construction of the Three Gorges Project, the annual observation was continuously undertaken by the Three Gorges gravity observation network established around the head area of the reservoir (the section between Sandouping and Badong). This network has pro-vided substantial high precise data related to seismic activity. However, the annual gravity survey together monitoring of the existed observation stations can hardly effective monitor some potential sei… 相似文献