I went ahead and opened up the environmental impact statement to see what they had to say. The things I noticed are summarized below. I'll repeat David Grinberg's disclaimer though: I am no kind of expert in environmental engineering or policy.
All page numbers are pages of the PDF, not the printed page numbers.
In some cases, the DAPL was found to have no significant environmental impact because without the pipeline, the alternative plans would have equal or greater impact. The point of comparison isn't "having a pipeline or doing nothing", it's more like "do we build a pipeline or do we transport all this stuff by train or truck?" In many cases, the pipeline is expected to have less impact (or the same impact) as the alternative plans.
For more information, see the ALTERNATIVES section on page 16. You could also look at the specific risk areas (3.1 is on page 23, 3.2 is on page 46, etc.).
Much of the discussion around the oil pipelines focuses on the possibility of oil spills. The environmental impact statement focuses a lot on risks of construction and maintenance. For example, the economic conditions section (page 3.8) describes the positive impact to the economy for building the pipeline.
Their risk management steps are in section 3.11 (page 100). Basically, they are going to construct and test the pipeline in according with industry best practices and federal law. Additionally, they have a mitigation and containment strategy in place in case there is an oil spill.
The list out the pieces of the risk mitigation plan on page 120:
BMPs designed to minimize the effects of construction on environmental resources;
Temporary and permanent erosion and sediment control measures;
Soil handling procedures designed to preserve the integrity of the soil (e.g., topsoil segregation,
Wetland and waterbody crossing and stabilization procedures
Wildlife and livestock mitigation measures
Restoration and revegetation procedures
Refueling and waste management procedures
Weed management procedures
Winter construction practices
Stormwater management procedures
The DAPL is being built along an already existing natural gas pipeline and an overhead electricity transmission line. These projects have already created some environmental impacts, reducing the marginal impact of the DAPL.
There are apparently geographic reasons that the environmental engineering staff thought were important. For example, in the aquatic resources section (page 80) they describe how the physical geography of the area limits possible harm to bodies of water and aquatic life. Some reasons include: the distance between the pipeline and waterways, the existence of sedimentary deposits which can buffer problematic chemicals, and an erosion control plan which will help keep the sediment in place.
When the pipeline does come near water, it is to be built deep enough and with thick enough pipes that the risk of spillage is reduced.
Finally, the report discusses the possibility that a pipeline could allow for growth in the oil/gas industry, which could create future risks to the environment (page 109). According to the state of North Dakota, the pipeline won't have this effect because it isn't a lack of pipelines slowing development.