They're basically synonyms (they mean the same thing), but I would add that "reach" implies knowledge of what time you or another person has left their starting point, on their way to their destination, and we know the person in transit will be getting there (arriving at) the destination at a specific time. "Arriving" at doesn't tell us anything about when the person left. Hope this helps...I've honestly never thought about it before. :)

If something is on the top shelf and we want to get it down, but we're not tall enough, we say that we can't reach. So, "reach" has a feeling of striving and attaining something that is at the end of our limits. Another example is when we talk about the "farthest reaches of the universe". It carries the idea of something that has gone as far as it can possibly go. Imagine you're reporting a world trip. You might say, "we arrived in Australia on Saturday, then we stopped in Indonesia on Monday, but we only stayed a day because we had to reach China in time to meet Susan. After that, we traveled on to Germany, arriving there on Friday. We took the trains all around Europe for the next week and then finally reached Uncle's farm in Egypt, and there we stayed until it was time to go home." The visits to China had a stronger sense of purpose and planning, so I used "reach". another example would be the question: "Do we have enough gas to reach DaLian?" You would never use "arrive" in this question.